A screen grab from Supersized Earth.

Brilliant programme with Dallas Campbell.  One sequence featured the question of how oil rigs can be moved. They're too big to be lifted by a crane. The solution is to use a boat which can submerge several metres below the surface of the sea; the rig is (very carefully) guided above it, then the boat rises to the surface with the rig on board. (See extra.)

I don't know why I find these things so fascinating. Maybe it's in the genes from my great grandfather who was a ship's captain from Ayr.