Come to think, the water below, decontaminated or no is going to be dirty and will degrade the integrity of the containment vessel even more. Now you would think to yourself that experts have thought this through, but I am amazed at some of the flub-ups they have had up to this point.

When the first significant leak into the sea was discovered, they tried sealing it with many things, including cement. Even I, who knows next to nothing, knew that concrete needs a certain water to other ingredients ratio in order to harden.* Well, they forgot that and only made sludge which hampered following efforts to plug the gap - though they did finally manage it.

*Believe I learned this from a Leave it to Beaver episode I saw as a kid; leave it to the beave.
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We are what we repeatedly do - Aristotle