The 50-year-old man, who has not yet been identified, was exploring at Bull Pot in Ingleton, yesterday, when the incident occurred.
Rae Lonsdale, duty controller from the Cave Rescue Organisation which attended the accident, explained that the man - thought to be an experienced caver - had been exploring an 80 metre deep pothole with a companion.
After abseiling around 65 metres down the pothole, the caver started to crawl along a passage - when a large block of limestone rock fell on top of him.
Mr Lonsdale said: “The limestone measured around one metre and a half, by one metre, by half a metre, and weighs a considerable amount - well over a ton.
“Unfortunately, the caver was just in the wrong place at the wrong time, the rock dropped down, hit him on the back of his neck and killed him instantly.
“His caving companion tried to help, he felt for a pulse on the arm that was free, but when he realised there was nothing he could do he came out of the cave and called the police.
“This rock has been there for thousands of years, and hundreds of other people have crawled underneath it without anything happening before.
“It’s a tragic accident.
“These incidents are very rare. Usually we just deal with trips and slips. We have dealt with fatalities before, and it’s very distressing.”
Members of the voluntary Cave Rescue team were alerted to the incident by police, and used various techniques to free the man, before removing him from the cave on a stretcher after the eight-hour operation.
Mr Lonsdale explained that the victim, from Lancaster, was believed to be experienced in caving as his equipment was worn but looked after properly.
A spokesman from North Yorkshire Police said: “Police were called at around 6.35pm on Thursday 12 February 2015, after reports of a caving accident at Bull Pot in Ingleton.
“A 50-year-old man from Lancaster was fatally injured by a falling boulder in the incident.
“The man’s body was removed from the scene by the Cave Rescue Organisation.
“Officers do not believe that there are any suspicious circumstances surrounding the man’s death and a report will be prepared on behalf of the coroner.
“The man’s family have been informed of his death, however we are not in a position to release his identity at this time.”