Flash back to 1944: It’s a misty Hawaiian morning and a military vessel carries a nervous crew and deadly cargo from Pearl Harbor into the Pacific. The crew’s instructions are clear: Travel eight kilometers out to sea and dump tons of unused chemical weapons that are piled on deck. As the ship reaches the open ocean, the captain slows the vessel and sailors start pushing their lethal freight into the water. During the next half-hour, several thousand chemical bombs go overboard and into the abyss.

Today, such a scenario seems unimaginable. But between the early 20th century and the mid-1970s, many nations used the deep ocean floor as a dumping ground for leftover bombs filled with chemicals such as chemical mustard, lewisite, sarin and tabun. Times — and mind-sets — were different then. The seafloor was considered an inaccessible place that no one would ever lay eyes on. And countries around the world faced a tough problem: Two world wars had left behind untold amounts of dangerous weapons that no one knew how to safely destroy.

Throwing this dangerous garbage into the deep sea seemed like a reasonable and safe solution. Between the end of World War I and 1975, when the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter came into force, the militaries of many nations dumped several hundred thousand tons of chemical weapons into various parts of the world’s oceans, including some 30 sites off U.S. shores, including the Mid-Atlantic and Hawaii. Since then, these weapons, along with the containers that hold them, have quietly rotted away on the seafloor. What that rotting means for the environment remains unknown.

Now a team of researchers at the University of Hawaii at Mãnoa is embarking on the most comprehensive study to date to examine a chemical weapons dump site in U.S. waters. The Hawaii Undersea Munitions and Material Assessment study, which is funded by the U.S. Army, will look for chemical weapons south of Pearl Harbor off the coast of Oahu. The work could help establish protocols for similar studies in the future, the researchers say.

The team’s target is one of at least three known chemical warfare dumping sites around the Hawaiian Islands. According to historical records, 16,000 chemical mustard bombs were dumped in the area. The study’s first objective is to find them, says Roy Wilkens of the Hawaii Institute of Geophysics and Planetology in Honolulu. He and his team surveyed a 60-square-kilometer patch of the seafloor using sonar technology last year. Their data generated maps that yielded some “target areas,” Wilkens says, that the team will explore with cameras, manned research submersibles and remotely operated vehicles in the coming months.

Looking for specific weapons, some of which are in corroded metal containers that are likely overgrown with vegetation, is tough because the seafloor around Pearl Harbor is anything but pristine. “The place is littered with everything from sunken ships to World War II airplanes,” Wilkens says. During a routine equipment test in the 1990s, he and his team even came across old American Standard toilets — apparently surplus left over after the war. It’s hard to tell a sonar blip generated by a container full of chemical weapons from that of other garbage, says Margo Edwards of the Hawaii Institute of Geophysics and Planetology. “So, whether our target areas are weapons or fields of toilets, we won’t know until we go down there with cameras.”

Once they locate the weapons, the team plans to test for degradation products — compounds that result from the decomposition of these chemical agents — in the surrounding water and sediment to determine whether the chemicals pose a threat to human health or the environment. Seawater has likely eaten away at the metal containers, letting the compounds seep into the environment. According to some reports, the metal boxes holding the weapons may not have even been completely sealed to begin with. “In order to make these containers sink, they actually shot holes into them so that they would fill with seawater,” Edwards says. “So between corrosion and the way these containers were handled at the surface there is certainly potential for leakage.”

At this point, the team plans to avoid assessing the actual weapons and will only sample the surrounding seafloor and water — in large part a safety precaution because the weapons material could still be very dangerous. Exactly what happens to nerve agents and other chemical weapons once they sit in cold ocean water for decades is still not known. Seawater can dissolve and dilute the chemicals, but laboratory studies show that some chemical reactions break down these substances into different compounds, says Peter Brewer of the Monterey Bay Aquarium Research Institute in Moss Landing, Calif. “I suspect that the areas that are actually contaminated are probably quite small,” he says.

Still, studies of a European dump site found that chemical mustard can form sticky nodules on bomb casings, trapping the intact chemical inside. “If you took those up to the surface and broke them, you could spill the mustard agent and cause serious burns,” Edwards says. This scenario happened in 1976 when three scientists surveyed a dredge dump off the coast of Pearl Harbor and got burned when they accidentally recovered chemical weapons.

The team will publish all study results, Wilkens says. That’s particularly important because the issue of chemical weapons in the oceans has been kept largely under wraps in many countries, Brewer says. “The topic has pretty much been a black hole.” This secrecy has resulted in the injury of some 500 unsuspecting individuals, mostly fishermen who captured rotting chemical weapons in their nets. And among ocean scientists, “there is a huge lack of awareness of this issue,” Brewer and Noriko Nakayama of the University of Tokyo wrote in a 2008 article in Environmental Science & Technology. Researchers routinely sample the water column in most of the known dumping areas, “often within a few meters of the seafloor” and “without regard for or knowledge of the disposed materials,” the duo wrote. In the United States, for example, ocean scientists have conducted thousands of studies in a 10,400-square-kilometer territory off the coast of California that contains seven known dumping sites, Brewer says. But no one knows exactly where the weapons are.

Over the past few years, the U.S. Department of Defense has reviewed historical records to better understand the location of all chemical weapons that the U.S. military disposed of in U.S. coastal waters. But information is spotty: The records usually don’t contain coordinates. In some cases, historical information only specifies “Atlantic Ocean” or “Pacific Ocean,” according to a 2007 Congressional Research Service report.

The Hawaii Undersea Munitions and Material Assessment study could be a first step toward establishing procedures for how to search for and characterize deep-sea chemical weapons dump sites, says Vicki Gaynor, vice president of Environet, a company that works with the University of Hawaii. Because this site lies at moderate depth, only about 500 meters below the surface, it “gives the researchers a chance to test their technology and see how effective it is at really identifying what’s out there,” she says. At this point, however, no one really knows what to expect or even what the risks of surveying the dump site are, so the research team will include hazardous materials experts from the U.S. Army’s Edgewood Chemical and Biological Center in Maryland. They will help design safety protocols, such as how to recover the submersibles and how to check them for any contamination before bringing the submersible on board or allowing anybody to get out. “We are on the cutting edge of doing this sort of thing,” Edwards says, “so we are going to be learning as we go.”