More than four decades after the U.S. halted a controversial ocean dumping program, the country is facing a mostly forgotten Cold War legacy in its waters: tens of thousands of steel drums of atomic waste.

From 1946 to 1970, federal records show, 55-gallon drums and other containers of nuclear waste were pitched into the Atlantic and Pacific at dozens of sites off California, Massachusetts and a handful of other states. Much of the trash came from government-related work, ranging from mildly contaminated lab coats to waste from the country's effort to build nuclear weapons.

Federal officials have long maintained that, despite some leakage from containers, there isn't evidence of damage to the wider ocean environment or threats to public health through contamination of seafood. But a Wall Street Journal review of decades of federal and other records found unanswered questions about a dumping program once labeled "seriously substandard" by a senior Environmental Protection Agency official:

• How many dump sites are there? Over the years, federal estimates have ranged from 29 to more than 60.

• How much of various types of radioisotopes are in the waste containers? While some isotopes are short-lived, others remain radioactive for hundreds or thousands of years.