Guillaume Nery dives at Dean’s Blue Hole, the deepest blue hole in the world.

This video blows my mind.

This is an example of a particular freediving discipline defined as:

Constant Weight Without Fins (CNF)
The freediver descends and ascends under water using only his own muscle strenght, without the use of propulsion equipment and without pulling on the rope. Constant weight without fins is the most difficult sportive depth discipline, because of absolutely no propulsing material to go down in the water. This category needs a perfect coordination between propulsing movments, equalization, technique and buoyancy.