Stalactites are formed by the precipitation of minerals from water dripping through the cave ceiling. The presence of stalactites tells us that this cave was once dry.
Mexican cave system Sac Actun is currently listed as the world's second longest underwater cave with 248 kilometres (154 miles) of mapped passage.
Cenote Taj Mahal
Late in the day rays of sunlight stream towards one of the cave entrances in system Taj Mahal in Mexico.
I encountered this tiny jellyfish, about the size of a fingernail, enthusiastically jetting about in the surface three metres of water at the Poor Knights Islands.