Every year across the UK the practice of "tombstoning" claims lives and causes serious injury.
A 20-year-old man suffered neck and back injuries in Southsea
The Maritime and Coastguard Agency (MCA) has repeatedly warned people not to take part in the practice - leaping into the water from a height - which it describes as "absolute madness".
Yet as England basked in the warm weather at the weekend, at least two people were seriously hurt in suspected tombstoning accidents.
Announcing details of one of the incidents, which happened at Sharrow Point in Devon, the MCA said "the madness had returned".
Mark Clark, from the MCA, said: "Coastguards have been using the term tombstoning for a very long time.
"We felt it accurately described the facts that rocks under the surface of the water resemble an English country churchyard.
"Or, you could be jumping from a very high tombstone and also, you don't know what you're going to hit on the way down."
Mr Clark said the practice had been prevalent for years, but the advent of websites such as YouTube, where footage could be posted online, had fuelled the craze.
He said: "With the advent of file sharing and video websites has meant that it's become much more prevalent, because people can look at each others' footage."
While there are no official statistics, Mr Clark said the MCA believed there were hundreds of tombstoning incidents every year, mostly perpetuated by teenage boys, but also on occasion older men fuelled by alcohol.
He said: "It is happening all over the place.
"We know from bitter experience of treating casualties that a couple of hours work for concerned lifeguards, coastguards and a helicopter crew can turn into a lifetime of pain for the individual and years of treatment from the NHS."