Much debris had to be cleared before the two bodies could be found and recovered from the 'basement'.
Cleaning up will be a long-term effort.

This led me to remember the basement complex of a tower complex built here. The basement level consisted of a number of cells, or 'rooms', some of which were open to house elevator bases, air circulation equipment and other appurtenances (such as the turbines in the reactor buildings), some walled off as unneeded, and merely part of the underpinnings of the tower complex.
I wonder if contaminated water could be leaking into something on the order of these 'cells', which would be hard to penetrate, but, if breached, could be culprits in water escape.
I don't have the plans of the Hitachi structure, so these musings have no support- just a passing thought.