Carlos Coste has just set a new breath holding world record yesterday by swimming for 492ft through a cave system in Mexico's Yucatan Peninsula using just a single fin and a torch. He did not have any breathing apparatus.
There is a video of the exploit here. If you look closely you can see the line he is following.
The swim took place in part of the Dos Ojos underwater cave system. Dos Ojos means Two Eyes and Coste swam through a tunnel linking two cenotes. The Yucatan Peninsula has hardly any surface water. All the water runs through subterranean tunnels. In places tunnel roofs have collapsed and created cenotes. A cenote is just an opening to the surface.
I have actually dived the route [post] that Coste took [but I was wearing full cave diving kit, not holding my breath]. Its a superb dive.