Youngsters jumping off a 100ft wall into the sea at Plymouth have been warned they are dicing with death.
Warm weather has re-ignited the practice of tombstoning at Plymouth Hoe in Devon where people as young as 12 have been seen diving.
Every year a number of people in the UK are killed or injured plunging into open water, a practice known as tombstoning.
Coastguards branded the stunts "complete and utter madness".
Fred Caygill, spokesman for the Maritime and Coastguard Agency, said: "We have all seen from past incidents the effects that this dangerous activity can cause.
"Death and serious injury can be a consequence of this type of activity, which not only has long term effects on the individuals concerned but also on family and friends.
"I would say to anyone thinking of undertaking this activity to think again. Death could be the possible outcome."
Peter Aley, assistant director of safer communities at Plymouth City Council, said the "long-standing problem" of tombstoners was a concern.
"We are aware of this issue and we are actively working on it."
In February, the council removed a 60ft (18m) platform on Plymouth Hoe which had been closed and caged in 2003.
Teenagers had continued to use it by climbing around the mesh cage.