A "blue hole" is a large, deep marine cavern or sinkhole that opens to the surface of the ocean
They are typically formed in carbonate bedrock like limestone or coral reefs
They often appear as a dark blue colour due to its depth, creating a distinct visual contrast against the surrounding water
Essentially, these are underwater sinkholes with vertical walls that can contain diverse marine life within their depths
They form when water seeps through cracks in the limestone, dissolving the rock over time and creating cavities that eventually collapse, forming a sinkhole
They vary in size, shape and depth, but most are ecological hot spots with a high diversity of marine life
The most well-known blue hole is the "Great Blue Hole" located near Belize in the Caribbean
It's a giant marine sinkhole that's part of the Belize Barrier Reef Reserve System