IIRC there was such an underground reactor project in a Nordic country. They hit a relatively minor problem, which due to the limited accessibility of the under

IIRC there was such an underground reactor project in a Nordic country. They hit a relatively minor problem, which due to the limited accessibility of the underground site could not be addressed quickly, resulting in reactor being damaged. Not saying its a bad idea overall, just noting an example. Other issues could be getting enough cooling in and out & avoiding ground water/aquifer contamination if it does indeed melt down.