This may be a naive question, but after the Fukushima Daiichi partial meltdown and studying the aftermath of Chernobyl it seems they could be helped by this idea.
In Chernobyl, the liquidators that cleaned up the disaster tunneled concrete under the reactor core and covered the whole complex in a big containment unit. Why could they not have a 30 meter pit below a reactor filled with water with a trap door holding the reactor up? This pit could be very thick reinforced concrete similar to a missile silo If there were a meltdown or imminent meltdown and there were no other options the reactor core could be dropped into this pit by triggering the trap door and then perhaps have lead shot dumped on it or sealed in concrete.
This seems very simple but there must be some reason why this is not practical. I know that the core would still be hot and it would still be a problem but would this not be better than it being exposed to air?