OAP. I am not really sure what advantage there would be to collapse a 5m deep reinforced concrete hole in the ground, it would still need back-filling but with the possibility of voids. As it was, several drums of STB were emptied into the pot to neutralize any residual mustard. My experience is +25 years, I believe some of the sites have been revisited.
Regarding deep sea dumping, the Beaufort Trench in the Irish Sea was a favorite spot. Ships were loaded at Cairnryan to proceed out; local knowledge will tell of the empty ships returning after a time that would need warp speed to reach the trench and back!