Please don't try this at home. Tombstoning is an extreme daredevil kind of an activity which involves diving into the sea or other water bodies form a very high platform, like cliffs. It is purely a recreational activity practiced by guys and gals with high adrenaline, and there are no records of tombstoning being practiced at a competitive level.
The reason the sport is given its name is because, the body's posture resembles a tombstone during the dive. On a funnier note, one can say that practicing the sport will lead you to your tombstone, and hence the name.
Jokes apart, it is a very dangerous sport that has resulted in several fatalities. Though tombstoning has be in existence for several generations, the attention the sport is getting is more recent, and it has not come in a positive light.
There have been over 20 fatal incidents and many times severe injuries reported in the last decade due to tombstoning, and has hence come under severe criticism. There have been some steps taken, like closing down certain locations, to discourage people from tombstoning. But not everyone is happy about such measures. There is a lot of support from several corners to keep the tradition going.