WWII’s Underwater Demolition Teams Paved the Way for the Navy SEALs | History


The so-called frogmen swam into enemy beaches unarmed, wearing only swim trunks, dive masks and fins.
The National Navy SEAL Museum

In the 1940s, most of the Pacific Ocean’s enormous underwater realm remained uncharted. Instruments and equipment were too rudimentary, the ocean too vast and much of the sea bottom too difficult to reach. Nautical charts showed the location of Pacific islands and atolls but offered little detail about the depths or underwater features of their beach approaches. Advances in sonar allowed United States ships to measure the ocean floor in deep water but not in the shallows surrounding land masses.

With no charts or technology available to guide vessels onto enemy shores, the job fell to the roughly 1,000 young swim scouts of the US Navy’s Underwater Demolition Teams (UDT), who were assigned to reconnoiter enemy beaches and clear coastal defenses ahead of Allied landings during World War II. These so-called frogmen swam into beaches unarmed, wearing only swim trunks, dive masks and fins. Training was conducted at the UDT’s Maui base, a facility consisting of basic tents, a ramshackle mess hall, cold open-air showers and smelly outdoor toilets.

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“It was not the way most people go to Hawaii,” says George Morgan, one of the war’s last surviving UDT veterans. Now 95 years old, Morgan joined the Navy in 1944 at age 17.

Relatively little known today, the frogmen’s legacy lives on in the Navy’s most elite special operations force. The SEALs inherited many techniques and traditions from their World War II predecessors, including demolition and underwater reconnaissance, stealth swim techniques, secret deployments, working in buddy pairs, and never leaving a man behind.


In Maui’s clear turquoise waters, swimmers like Morgan—a former lifeguard from New Jersey—learned how to survey enemy beaches, use explosives to clear underwater obstacles and blast coral reef to create boat channels. Forty percent of recruits failed to make it through training. Some became panicky on the open sea, while others lacked the endurance for long-distance swimming or the skill to swim the UDT way: in stealth.

To avoid detection by the Japanese, UDT men were taught to swim without making a splash. Morgan learned how to rely on the sidestroke and breaststroke, so his legs and arms never broke the surface of the water. The UDT men also practiced turning their heads when they swam to avoid their masks reflecting the sunlight. They learned to never come up for air on the crest of waves, only in the troughs between them.

A UDT man rigging an enemy obstacle replica

A UDT man rigging an enemy obstacle replica

The National Navy SEAL Museum

The men’s specialized training first came into use ahead of the Battle of Saipan in 1944, when 200 swimmers were assigned to measure the water depth of the beach approaches and scout for enemy mines and coastal defenses. The daylight mission took place on June 14, a full day before the US invasion. The UDT men coated themselves in blue paint to provide camouflage in the lagoon. Working in “buddy pairs,” they dropped weighted fishing lines to the seafloor to collect depth measurements, which they recorded on plexiglass slates tied to their knees. In shallow water, they transformed their bodies into yardsticks, painting black lines every 12 inches on their necks, torsos and legs.

During operations on Saipan and other Pacific islands, the UDT men realized that enemy bullets slowed down and began sinking a few feet beneath the ocean surface, allowing the swimmers to hold their breath and dive underneath them. Some swimmers caught the sinking bullets in their fingers and tucked them into their pockets as souvenirs. Others later drilled a hole through the sniper bullets, tied them on strings and wore them as necklaces.

A wartime photo of George Morgan

A wartime photo of George Morgan

Courtesy of George Morgan

At the time, scuba equipment was still in its fancy. A snorkel would have slowed the swimmers down, so the UDT brought in a Hawaiian pearl diver to teach the men how to hold their breath underwater.

Pearl diving has been practiced for more than 2,000 years across the Pacific. In search of their treasure, pearl divers have been known to stay underwater for up to seven minutes. They do so by entering a state of semi-hibernation. The body’s natural instinct is to panic when deprived of oxygen, but pearl divers practice relaxing their bodies, making their blood pressure and heart rate drop.

To encourage healthy competition between the men, officers liked to stage contests. Holding one’s breath became a popular contest at the UDT’s Maui base. Morgan could last 2 minutes and 45 seconds. Few men could go beyond four minutes without blacking out, but one swimmer clocked a record of 5 minutes, 5 seconds.


Due to the pioneering nature of their work, the UDT men were often forced to improvise and invent new equipment. One man designed the reel of fishing line that was used to conduct depth measurements by welding powdered milk cans end to end and fitting them with buoyant wooden flanges. In a field experiment, another man devised a clever way to keep fuses, firing caps and matches dry underwater by wrapping them in a condom. (The trick worked so well that it became standard practice. The men were soon consuming condoms by the thousands for their demolition work, to the astonishment of the base’s supply personnel.)

UDT men swimming in stealth toward a row of beach obstacles

UDT men swimming in stealth toward a row of beach obstacles

The National Navy SEAL Museum

A third UDT member came up with a clever way to prevent dive masks from fogging up: spitting into the mask, then swirling the saliva around with ocean water. Decades later, the tactic would become standard practice among sport divers.

It was difficult for the UDT to obtain equipment in bulk. Glass-windowed rubber masks were used mainly for spearfishing, which was then a niche sport. Only a few pairs of masks could be found in Hawaii sports stores. Then an officer saw an advertisement for the masks in a US magazine. An urgent dispatch was sent to the sporting goods company, and the store’s entire stock was flown to Maui in secret.

Rubber swim fins were just as rare. They were first produced in the US in 1939, by an American swimming champion in Los Angeles named Owen Churchill. On a visit to the island of Tahiti, he’d watched local boys swimming with rubber fins reinforced with metal bands. (The fins in Tahiti were made from crepe rubber, which is tapped from trees such as the Pará rubber tree.) He tracked down a French inventor who’d designed his own pair, negotiated a license to make them in the US and sold 946 pairs in 1940. Largely thanks to the UDT, Churchill sold more than 25,000 pairs of fins during the war. If a lost fin washed ashore on an enemy beach, the Japanese could find the name of the odd webfoot contraption stamped on the rubber—“Churchills”—along with its creator’s address in Los Angeles: 3215 W. 5th Street.

UDT men returning from a successful demolition mission off Saipan

UDT men returning from a successful demolition mission off Saipan

The National Navy SEAL Museum

The UDT men also needed massive amounts of fishing line. For his first order, the chief instructor of the Maui base dispatched one of his officers to Pearl Harbor to obtain 150 miles of it. The supply officer at Pearl Harbor looked hard at the UDT officer. “I thought we came out here to fight a war, and you men at Maui are out fishing,” he said, shaking his head. “What kind of fishing is it where you need 150 miles of line?”

“Japanese fishing,” the officer replied.

A UDT member attaching explosives to an underwater obstacle

A UDT member attaching explosives to an underwater obstacle

The National Navy SEAL Museum

In addition to the UDTs’ vital reconnaissance and demolition work, Morgan believes the unit provided a confidence boost for assault troops, a reassurance “that they weren’t going in blind, that they knew what they were about to face.”

UDT swimmers participated in almost every major amphibious assault of the Pacific Theater. The frogmen braved fierce enemy fire and rough seas as they scouted the bullet-swept lagoon of Saipan, the black sands of Iwo Jima, the shark infested reefs of Okinawa and even the cold waters of Tokyo Bay.

GIs called them “half fish, half nuts.” Today, we call them Navy SEALs.

Adapted from Into Enemy Waters: A World War II Story of the Demolition Divers Who Became the Navy SEALs by Andrew Dubbins. Copyright © 2022 by Andrew Dubbins. Available from Diversion Books.

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