Free diving
Free diving is about not having tanks or air.
The world record deepest free dive is 281m with fins.
The longest time under water was over 11 min.

Submarines
Submarines are basically are big tanks of air.
They can go deeper than any divers.
Periscopes let them see above the surface.

Tunnels under the sea
The longest under water section is the Channel Tunnel connecting France and England.
Some of the Cornish mines go right under the sea.
Some people think there will be cities under the sea in the future.

Self contained under water breathing apparatus
Scuba diving is a type of diving unlike free diving scuba diving you carry on your back a tank of air.
Divers that go to deep could get the bends.
If you get the bends you have to go in a special machine that takes away the bends, called a decompression chamber.