Away from the cavern zone, underwater caves exist in perpetual darkness, lit up for mere seconds as cave divers pass through.
A slab of rock that was once part of the ceiling lies like a giant altar on the road through Mexican cave system Sac Actun.
The world explored by cave divers
In the subterranean passages of Mexico's Yucatan Peninsula is an underwater world enjoyed by cave divers.
Eden - the River Run
Cooler fresh water sits on top of warmer salt water, that intrudes into Mexico's caves from the sea. The halocline is what we call this meeting between the two bodies of water.
Inside the Canterbury wreck
She's filling up with silt and Bigeyes. They're nocturnal fish that use the bowels of the Canterbury for shelter during the day and venture out to feed at night.
Ten years have passed since the ex-Navy frigate of the HMNZS Canterbury was scuttled in Deep Water Cove in the Bay of Islands.
Diving for science
Teams of cave divers coordinate efforts to collect samples for scientific researchers.
A vibrant world of black coral trees and sponges thrives on New Zealand's deep reefs as you descend beyond the level at which there is sufficient light for kelp to grow.
The rocky reef at 60 metres
The skeleton of an old black coral tree smothered in anemones - there's a lot of life worth investigating at 60 metres on the rocky reef in New Zealand.