What Penny S was talking about is a Sussex "dew pond", generally built high up on the South Downs (chalk hills) in Sussex, UK.

This is a circular excavation in the chalk, lined with impervious material like clay. They have no water source at all, but rely on the fact that the air at the top of the hill is cooled by altitude and as water condenses as dew it is trapped by the clay and accumulates over time to form a pond. Also, I suppose, they get filled by rain water from time to time. Probably in the Google somewhere.