Digging a hole - walls under ground.

[magnetaress]

Probably a very old sewage drain. We still have those made of actual terracotta pipe here. 60+ year old structure. Still in use. Still got spare pipe laying about too. It's how we know. We also looked for it in the ground.

Run water from the house and see if it floods.

On the bright side, it's already excavated if its important.

May have been disused, obsolete, ours are still on the planning maps at cityhall from back then. Got to order that up in person and pay cash to look at them tho. Not stuff on the web.