There are probably more than 14 million kilograms of explosives on the seabeds off US coasts.
The unexploded bombs – dumped there by the military – pose a continuing danger as energy companies drill for oil underwater.
Most of the debris consists of malfunctioning or outdated ordnance or weapons and explosives seized from war enemies.
It was dumped from the 1940s, when there was little regulation over international water, until 1972, when an international treaty banned uncontrolled waste disposal.
"These bombs are a threat today and no one knows how to deal with the situation," said William Bryant, a Texas A&M University professor of oceanography who recently presented the findings at a conference aimed at tackling the problem.
"If chemical agents are leaking from some of them, that's a real problem. If many of them are still capable of exploding, that's another big problem."
The risk of unused bombs suddenly detonating is small, but any accident can be catastrophic, particularly if there are pipelines nearby.
Bryant is even more worried about toxic agents, such as mustard gas, the dreaded World War I weapon, which is carcinogenic and causes burns.
It may harm marine life, and has on several occasions been accidentally lifted off the sea bottom by US fishermen.