Hi. First is your house built before 1935. If not, it’s virtually guaranteed that you won’t have any structural elements below the soil. It’s possible that was a land drain for a greenhouse, or shed, it’s the right shape and I have one in mortar lined brick that leads to an underground water storage chamber, which is made of brick and is built up on a small bank. My house has a kitchen garden, which is next door to a big house, and used to be part of the estate. It’s a gravity fed watering method, uses big terracotta piping. It’s fascinating…. But as I said, you need to be in an old environment, where the land hasn’t been churned up. If you want to go exploring, use a metal rod and knock it into the ground around the area, it’s going to be obvious if you have anything substantial down there. The local council had records of what houses had a shelter, you could certainly get exploring