Guam: A Biogeographic and Maritime Cultural Landscape Exploration of a World War II Amphibious Battlefield

Past Expedition

Dates

January 27-February 25, 2023 // July 27-August 23, 2023

Location

Pacific Ocean

Overview

In 2023, a team of researchers conducted fieldwork on Guam in War in the Pacific National Historical Park to further our understanding of one of the most significant invasions of the World War II (WWII) Pacific theater as well as how it may have shaped the natural environment in the area today.

Guam was a strategic military location for both the United States and Japan during the war. And, in July 1944, U.S. troops invaded and secured the Japanese-held island.

During this two-part project, the research team used ship-based instruments, an uncrewed surface vessel, and divers to identify, map, and document submerged resources related to the invasion (e.g., shipwrecks, aircraft, amphibious vehicles, and artillery). They also collected data to assess war-related impacts on the coast, seafloor, and barrier coral reef to investigate how damage to the reef may have impacted the island’s ecosystem and coastal vulnerability.

The project study areas for Guam: A Biogeographic and Maritime Cultural Landscape Exploration of a World War II Amphibious Battlefield are the landing beaches of Asan and Agat, shown here. *Map courtesy of National Park Service.*
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Part 1: January 27-February 25, 2023

From January 27-February 25, 2023, a team of researchers exploring Guam’s World War II (WWII) underwater battlefield located and documented blasting sites and identified over 280 targets that warrant further investigation.

Anne Wright diving on an Amtrac amphibious tractor as part of an effort to generate high-resolution, in-depth documentation of this important cultural resource, one of the few currently known underwater relics from the 1944 invasion of Guam. *Image courtesy of National Park Service via Guam: A Biogeographic and Maritime Cultural Landscape Exploration of a WWII Amphibious Battlefield.*
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Anne Wright and Tahzay Jones preparing a Trimble SX10 scanning total station to collect high-density, high-accuracy elevation data along the coastline in the Agat unit of War in the Pacific National Historical Park. *Image courtesy of National Park Service via Guam: A Biogeographic and Maritime Cultural Landscape Exploration of a WWII Amphibious Battlefield.*
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Orthomosaics showing a crater created by a U.S. underwater demolition team (UDT) blast to remove a Japanese obstacle on the reef flats of Asan Beach (top) and a blasting area created by UDTs off Agat Beach to land tank landing ships (LSTs) (bottom). *Images courtesy of National Park Service via Guam: A Biogeographic and Maritime Cultural Landscape Exploration of a WWII Amphibious Battlefield.*
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This fieldwork was done during the first part of a two-part project. The objectives of the project are to inventory submerged material (including shipwrecks, aircraft, amphibious vehicles, artillery, and other artifacts) related to the 1944 invasion of Guam and to study how WWII-related activities have altered the coral reef ecosystem and coastal processes on Guam today.

The most exciting result of this winter’s fieldwork was the unexpected discovery of a clear line of blast craters inside the barrier reef off Asan. The locations of these craters correspond with the positions of Japanese obstacles destroyed by U.S. underwater demolition teams (UDTs) shown on a historic map. Other evidence of blasting by UDTs included a 200-foot area of the barrier reef off Agat that enabled tank landing ships (LSTs) to beach to offload cargo. To document the sites, the researchers used photography, photogrammetry, a multibeam sonar system on an uncrewed surface vessel (USV), and high-precision Global Navigation Satellite System (GNSS) equipment.

This blast damage to Guam’s offshore environment, 80 years later, is a reminder of the military activities that took place there and provides evidence that such activities can have lasting environmental impacts. To search for indications of potential blast-related coastal impacts (e.g., erosional hot spots) and to establish a baseline for future comparison, the team also used the GNSS equipment and a scanning total station to conduct coastal elevation surveys.

In addition, using vessel-based side-scan sonar and a magnetometer, the team found almost 250 targets. While some of these targets may be unrelated man-made objects or natural features, the amount of data collected and the supporting archival research suggest the likelihood of discovery of cultural resources is high. Data were also collected in shallower waters with the USV and its multibeam sonar system. These data are still being analyzed for additional targets. This summer, divers will closely investigate and document each individual target.

Today, the landing beaches of Asan and Agat are part of War in the Pacific National Historical Park. Documenting the remains of the battle and understanding the impacts of erosion and sea level rise that the park is now experiencing — and can anticipate in the future — will help park managers preserve and manage its resources (both submerged and on land).

Ultimately, this project will expand our knowledge of one of the most significant invasions of the Pacific Theater — and a turning point in WWII — both from historical and ecological perspectives.

Check out the story map to learn more about the first part of this project.

Published March 30, 2023

Part 2: July 27-August 23, 2023

Researchers studying Guam’s World War II (WWII) underwater battlefield returned to the invasion beaches of Asan and Agat between July 16 and August 23, 2023. During their fieldwork, they investigated potential submerged cultural resources, measured coastal elevation, and interviewed local residents.

Searching for WWII Cultural Resources

During this visit, divers investigated 161 magnetometer anomalies and side-scan sonar targets in deep water outside the barrier reef off Asan and Agat beaches that were identified during remote sensing operations in February 2023. Some of the targets turned out to be modern debris and trash, but many proved to be underwater remnants from WWII.

Divers prepare for a dive in the Asan unit of War in the Pacific National Historical Park. *Image courtesy of National Park Service via Guam: A Biogeographic and Maritime Cultural Landscape Exploration of a WWII Amphibious Battlefield.*
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Among the targets was a significant amount of unexploded ordnance (UXO). Some of this UXO was at a known post-war dump site in the water off Asan Beach. This site had previously been surveyed to a depth of 150 feet, but this team investigated targets up to 200 feet in depth, enabling the identification and documentation of previously unknown UXO in the park’s deeper waters. They also found UXO (artillery rounds and mortar shells) outside of the dump site, meaning they may have been used in battle.

A World War II-era jeep trailer loaded with vehicle parts and equipment see off Agat Beach.

A World War II-era jeep trailer loaded with vehicle parts and equipment seen off Agat Beach. Image courtesy of National Park Service via Guam: A Biogeographic and Maritime Cultural Landscape Exploration of a WWII Amphibious Battlefield.
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In addition, the team investigated and documented a WWII-era jeep trailer loaded with vehicle parts and equipment, Marston Mats, and what they believe is a Japanese fishing boat. A local diver familiar with the boat shared photos of it from the 1980s and early 2000s, which the team hopes to use to learn more about the boat and track changes to it over time.

Inside the barrier reef at Asan, the team searched for additional cultural resources. During this walking survey, they found 171 artifacts that may be related to the war, including spent artillery rounds, brass/copper fragments that may have come from artillery rounds and mortar shells, pipe fragments, and Japanese pottery shards.

A piece of broken Japanese pottery found during the walking survey inside the barrier reef at Asan Beach (with scale bar to show size). *Image courtesy of National Park Service via Guam: A Biogeographic and Maritime Cultural Landscape Exploration of a WWII Amphibious Battlefield.*
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Measuring Coastal Elevation

The team also continued collection of high-resolution elevation data. They used a Trimble SX10 scanning total station and R12i global navigation satellite system (GNSS) equipment to collect location and elevation data along the coast and generate a detailed “picture” of the landscape.

A project team member collecting high-resolution coastal elevation data using a Trimble SX10 scanning total station. *Image courtesy of National Park Service via Guam: A Biogeographic and Maritime Cultural Landscape Exploration of a WWII Amphibious Battlefield.*
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They will use these data to develop a holistic land-to-ocean understanding of the study areas. Specifically, they seek to understand if WWII-related activities, such as the blasting of the reef, may have altered Guam’s coral reef ecosystem and coastal processes, like erosion that can make the island and its inhabitants more vulnerable to storms, like May’s Category 4 Typhoon Mawar, and sea level rise.

Members of the team were on Guam when Mawar struck, and they photo-documented the study areas immediately before and after the storm and used GNSS equipment to map the erosion it caused. Among the storm impacts were shoreline and bank erosion, redistribution of sand along the beaches and intertidal zones, and reconfiguration of the Asan River mouth. By comparing data collected this summer to the data collected the previous winter, the team will be able to visually and quantitatively assess the short-term coastal change caused by the storm and consider how that change may have been influenced by WWII impacts to the reef.

A project team member documenting bank undercutting and erosion along the shoreline at the Agat unit of Guam’s War in the Pacific National Historical Park immediately after Typhoon Mawar. *Image courtesy of National Park Service via Guam: A Biogeographic and Maritime Cultural Landscape Exploration of a WWII Amphibious Battlefield.*
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Conducting Oral History Interviews

To support the other elements of this project, the team also interviewed residents from the village of Asan, adjacent to Asan Beach. Questions focused on past and present community use of the reef; their opinion on how the war may have affected how the community uses the reef; and undocumented submerged WWII artifacts.

Orthomosaic of a submerged Amtrac (amphibious tractor) off Asan Beach. The research team learned about this site through the oral history interviews and subsequently documented it. *Image courtesy of National Park Service via Guam: A Biogeographic and Maritime Cultural Landscape Exploration of a WWII Amphibious Battlefield.*
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During these interviews, they learned about a section of the reef that collects significantly more trash than elsewhere on the reef. Interestingly and unbeknownst to the interviewees, this site is a blasting area created by U.S. underwater demolition teams off Agat Beach to provide access to tank landing ships (LSTs). They also learned about, and subsequently documented, a submerged Amtrac (an amphibious tractor) off Asan Beach that had been minimally documented and tracks embedded in coral that may have come from American equipment deployed during the invasion.

Tracks embedded in coral off Asan Beach that may have come from American equipment deployed during the World War II invasion of Guam. The research team learned about these tracks through the oral history interviews. *Image courtesy of National Park Service via Guam: A Biogeographic and Maritime Cultural Landscape Exploration of a WWII Amphibious Battlefield.*
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Bringing It All Together

With the conclusion of the fieldwork, team members are now analyzing their data to produce 3D photogrammetric models of artifacts, reconstruct the battlescape, and assess the impacts of battle-related reef damage. Along with technical reports and maps, the team will produce outreach and education materials for the public based on project findings.

Ultimately, this project will contribute to the nation’s military and maritime history, help us understand how war-related activities may have shaped the natural environment in the area today, and illustrate the importance and evolution of Guam’s maritime culture at Asan and Agat beaches. These outcomes will inform federal resource management decisions (e.g., use of the area and conservation and protection of artifacts) as well as future research on human impacts to coral reefs and how reefs affect coastal resilience.

Published October 12, 2023

View Expedition Overview

Using Oral History to Understand Impacts of World War II Reef Destruction on Guam Today

There have been several oral history projects completed in collaboration with Guam's CHamoru community, but none of them focused solely on maritime traditions and uses of the reefs. As the National Park Service Scientists-in-Parks intern for the exploration of Guam's World War II amphibious battlefield, I helped oversee an oral history project focusing on the reef landscapes within War in the Pacific National Historical Park: how these landscapes have been used throughout different periods in Guam's history and how they may have been affected by World War II.

October 3, 2023

Erosion and bank undercutting along the shoreline at the Agat unit of Guam’s War in the Pacific National Historical Park immediately after 2023’s Typhoon Mawar.

Measuring Elevation to Understand Coastal Change

From January 27-February 25, 2023, a team of researchers will begin their search for submerged shipwrecks, aircraft, amphibious vehicles, artillery and other artifacts related to the 1944 invasion of Guam during World War II.

September 18, 2023

U.S. soldiers unloading supplies on a Pacific Island beach during World War II with pierced steel planking (PSP) in the sand in the foreground.

Project Findings Once "Paved" the Way for Invasion

While investigating previously identified side-scan sonar and magnetometer targets in the waters off Guam’s World War II (WWII) landing beaches, technical divers from the National Park Service Submerged Resources Center and the NOAA Diving Program discovered five sections of pierced steel planking (PSP), also known as “Marston mats,” off Agat beach.

August 31, 2023

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Mikey Kent with the NOAA Diving Program completes an underwater circle search for a magnetic anomaly using a metal detector.

Technical Divers Enable Greater Access to Our Past

In July 2023, the project team returned to Guam to investigate previously identified targets. As part of this effort, a technical diving team composed of divers from the National Park Service Submerged Resources Center and the NOAA Diving Program spent a week investigating deepwater targets using closed circuit rebreathers.

August 17, 2023

Documenting Underwater World War II Blast Artifacts

In 1944, U.S. underwater demolition teams used explosives on and around Guam’s barrier reef to aid the U.S. invasion of the Japanese-held island. Almost 80 years later, during this 2023 project, our project team located and documented some of the blast sites off the landing beaches of Asan and Agat.

April 11, 2023

Among the gear the National Park Service research team is using to document the seafloor of Guam’s underwater battlefield is SeaArray, a multi-camera photogrammetry platform.

Exploring Guam’s World War II Amphibious Battlefield

Shortly after the December 7, 1941, Japanese attack on Pearl Harbor, the Japanese invaded the island of Guam and quickly seized control from a small number of American forces and the Guam Insular Force Guard who were unprepared for the attack.

February 21, 2023

Expedition Overview

From January 27-February 25, 2023, a team of researchers will begin their search for submerged shipwrecks, aircraft, amphibious vehicles, artillery and other artifacts related to the 1944 invasion of Guam during World War II.

January 20, 2023