It should be a boat
https://en.wikipedia.org/wiki/Mariana_Trench#Possible_nuclea...
ie we know nothing much about that area... who's to say it wont suddenly turn up on a beach or in fish or plant life after 50 years of being down there...?
erm, the literal 10 kilometers of water above them?
Also Mariana Trench is called a trench because is a trench. A trench 10 km deep.
We dont know what we dont know... what else do we not know?
Furthermore, plate subduction zones are associated with very large megathrust earthquakes, the effects of which are unpredictable for the safety of long-term disposal of nuclear wastes within the hadopelagic ecosystem.[53]
(Premise of The Meg ***SPOILER*** is megalodon sharks survived in the Mariana Trench)