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

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

Documenting Underwater World War II Blast Artifacts

In 1944, U.S. underwater demolition teams (UDTs) 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.

During World War II (WWII), Japanese forces attempted to deter U.S. troops from an amphibious invasion of Guam, and other islands in the Pacific, by placing obstacles in the water to prevent American landing craft from easily reaching shore to deliver soldiers directly to the beach. An after-action report describes the obstacles offshore of Asan Beach as being “in an almost continuous front along the reef…these obstacles were piles of coral rock inside a heavy wire frame made of heavy wire net” (UDT Report 1944). Offshore of Agat Beach, the obstacles were placed “all along the reef, built of coral rock, enclosed in a light wooden frame” (UDT Report 1944). The UDTs used upwards of 10,600 pounds of tetrytol explosives to remove these obstacles and sections of the barrier reef to clear direct paths to the beaches for American landing crafts.

Hand-drawn historic map found in the records of Underwater Demolition Team (UDT) #3 that indicate the locations of obstacles placed by Japanese forces that were removed by UDT #3 at Asan Beach. The small Xs in the light blue area indicate an obstacle.
Hand-drawn historic map found in the records of Underwater Demolition Team (UDT) #3 that indicate the locations of obstacles placed by Japanese forces that were removed by UDT #3 at Asan Beach. The small Xs in the light blue area indicate an obstacle. Image courtesy of National Archives at College Park: Action Report UDT #3 Forager Operation, Record Group 38: Records of the Naval Office of the Chief of Naval Operations, World War II Action and Operational Reports, Records of the Navy, Declassification Authority: NND968133. Download largest version (jpg, 4.1 MB).

Prior to the fieldwork, we conducted archival research to determine where the Japanese obstacles had been placed. Among our finds was a hand-drawn historic map of Asan Beach that indicated the location of obstacles inside the reef that were destroyed prior to the invasion. In the field, we located a clear line of craters across the reef flat in the vicinity of the blasting positions on the historic map. The presence of these blast craters, nearly 80 years after the events of WWII, was an unexpected (and exciting!) find for us. The craters exist in a very shallow, fairly high-energy environment, so we expected that they would have been filled in by now (e.g., with sand and coral fragments) through natural coastal processes.

We made a photogrammetric model of the deepest crater and photographed the others. In addition, we documented the line of craters with our uncrewed surface vessel equipped with a multibeam sonar capable of collecting high-resolution data, and we also recorded the location of each crater using high precision Global Navigation Satellite System (GNSS) equipment.

Orthomosaic showing a crater created by a U.S. underwater demolition team blast to remove a Japanese obstacle on the reef flats of Asan Beach.
Orthomosaic showing a crater created by a U.S. underwater demolition team blast to remove a Japanese obstacle on the reef flats of Asan Beach. Image courtesy of National Park Service via Guam: A Biogeographic and Maritime Cultural Landscape Exploration of a WWII Amphibious Battlefield. Download largest version (jpg, 718 KB).
Orthomosaic showing a blasting area created by U.S. underwater demolition teams off Agat Beach to land tank landing ships (LSTs).
Orthomosaic showing a blasting area created by U.S. underwater demolition teams off Agat Beach to land tank landing ships (LSTs). Image courtesy of National Park Service via Guam: A Biogeographic and Maritime Cultural Landscape Exploration of a WWII Amphibious Battlefield. Download largest version (jpg, 745 KB).

At Agat, our analysis of historic reports and maps allowed us to identify the location of a channel blasted through the reef. Here, UDTs deployed thousands of pounds of tetrytol through a 200-foot area of the barrier reef to essentially create a ramp where three tank landing ships (LSTs) could beach side by side to offload cargo. We mapped the area using the National Park Service’s SeaArray photogrammetry system and created a 3D model from the data. It was interesting (and rather incredible) to see coral rubble from the actual blasting is still present within the channel.

These findings really brought to life the reason and meaning for strategic military activities that took place during WWII. And, we’re still learning about the lasting influence that those activities have had, and continue to have, on the coral reef ecosystem and coastal processes on Guam today.

By Monique LaFrance Bartley, National Park Service Ocean and Coastal Resources Branch
Published April 11, 2023