ᐅ Basement floor has cracks/bulges

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 slightly damp from mopping. Apart from that, there are no damp spots on the floor or the walls.
During heavy rain, some rainwater occasionally comes in through the basement door, which is now being replaced. However, the bulge is located in a different room.

What could this be? Does anyone have an idea?
Best regards,
timwille91

Hello Tim,
It is always less than ideal to assess something based on a photo. However, it appears that this area may have been modified afterwards. Maybe something was installed there before? Are there any old plans available? In any case, the area should be chiseled out until sound concrete is reached and then professionally re-concreted. Before that, measure the moisture to ensure that no dampness is coming from below. In the 1950s, it is very unlikely that any vapor barrier or membrane was installed.

It looks like there is simply a floor drain there.
However, it is somewhat rusty. That would explain the cracks and bulge for now.