CTD stands for conductivity, temperature, and depth, and refers to a package of electronic instruments that measure these properties. The CTD is an essential tool used in all disciplines of oceanography, providing important information about physical, chemical, and even biological properties of the water column.

In this image, taken during the Beyond the Blue: Hawai‘i Mapping expedition on NOAA Ship Okeanos Explorer, the team prepares to conduct CTD operations. Image courtesy of Caitlin Bailey, Global Foundation for Ocean Exploration, 2024 Beyond the Blue.