Basement / Civil Engineering - What kind of shaft is this?

But the rainwater would seep away, or am I wrong?

If it was a cesspit, it would have to be sealed, a soakaway of course not, test with rainwater

Presumably, this is a multi-stage clarifier. Stage 1 solids. Stage 2 suspended particles. Stage 3 infiltration???? Introduce rainwater into the first stage then you will see what happens.

Actually a pity, I would have found access to a burial chamber, a labyrinth, or a bunker facility much more exciting...