"Between 1971 and 1982, more than 200,000 drums of radioactive waste were dumped by several European countries in the Northeast Atlantic, at depths reaching over 4,700 meters . The exact location of these drums and, above all, their potential impacts on the deep-sea environment remained largely unknown until studies conducted in the 1980s"
Inventory:
https://www-pub.iaea.org/MTCD/Publications/PDF/TE-1776_web.p...
Articles:
https://www.ens.psl.eu/en/node/9080
https://www.sciencealert.com/150000-tons-of-radioactive-wast...