I was digging a flower bed in my garden and as i put the fork into a particular area the earth fell away and a cavern / well appeared. It goes down about 2 metres where there is a load of earth and until that point it has circular bricked walls. This wasn't on my deeds when i bought the house nearly four years ago.

What should i do with it? Could it be the mains water junction for the local area? Should the council cap it for me or do i have to pay to get it done?