Deepdale Pond in Deepdale Wood near Yealand Redmayne in Lancashire.
This area was once an underground cave, but the roof collapsed, leaving a deep hollow called a doline. Water collected in the doline, creating a pond. It was much bigger before 1916, but an explosion at a Morecambe munitions factory caused shock waves which disturbed the underlying rocks and much of the water drained away.
Formerly called Lily Pond, it was a popular site for picnics up until the explosion.
The area is now bog.and part of an SSSI (Site of Special Scientific Interest).