Guessing that the amount of urea / perchlorate dust/residue on board still made it a severe explosion risk even after offloading the material. I’d assume this stuff wasn’t in hermetically sealed packaging with the utmost care in storage and containment.

A small vessel on the ocean floor is not a big deal

Leaving it drifting would be a much bigger hazard to both chips and marine life, for when it strays into a crude oil tanker and causes an ecological disaster

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