Basement floor has cracks/bulge

Hello dear forum!

We have bought a house and in the basement there is a "bulge" with cracks visible. It sounds hollow when tapped. The house was built in 1952 and has a full basement.
In the picture, the floor is currently wet from mopping. Otherwise, there are no damp spots on the floor or walls.
During heavy rain, some rainwater does come through the basement door occasionally. However, it is being replaced now. The bulge is located in another room though.

What could that be? Anyone have an idea?
Best regards,
Tim

Hello Tim,
judging from a photo is always suboptimal. However, it looks like this area was reworked afterwards. Maybe something was there earlier? Are there old plans? In any case, chisel out the area until you reach stable concrete and professionally re-concrete it. First, measure the moisture to ensure that nothing is pressing from below. In the 50s, they certainly did not install any foil.

Seems like there is just a drain/gully there. But it is a bit rusty. But that of course explains the cracks/bulge for now.