I've had the same problem, but I somehow managed to get out from underneath the foundation while fighting the shark and moving around
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I destroyed two foundations in the middle with 3 water purifiers, 3 cooking stations and a pillar connected to a few floors on them , and now I can't place new ones. I can fall in the hole though, and this only goes for these two spots. It's like the foundations around those two don't even exist, even though in can walk on them and place things on them just fine.