Oceans of Nuclear Waste!
Between 1946 and 1993 a bunch of countries dumped 85 PBq* of radioactive waste into the ocean. Slurries, solids, reactors - you know, the stuff you find lying around and just want to put in the ocean.**
It was terrible, and it seems we've mostly stopped it. But that shit is still down there... So we explore!
* I don't really know how much a PBq is, but it's a lot more than 'nothing', and 'nothing' is the correct amount of radioactive waste that should go into the ocean
** No, you don't find it lying around and you don't want to put it in the ocean, you're a good person listener.