Blue Holes

A blue hole is a circular, steep-walled depression developed in a bank or island that is open to the surface. It is akin to a marine sinkhole and is composed of carbonate bedrocks. A blue hole descends hundreds of kilometers and connects to mysterious tunnels.

The Chinese believed these portals to be entrances to an underground palace ruled by dragons.

The Mayans had an interesting idea that these blue holes were the gateways to the underworld.

The expedition of OceanX in 2018 took the plunge into the blue hole of Belize and explored the many mysteries that lie beneath. The entire expedition was also streamed live on Discovery.

https://go.discovery.com/tv-shows/into-the-blue-hole/full-episodes/great-blue-hole-of-belize

Most blue holes were formed from sinkholes that slowly developed over time.

The creation of blue holes began many thousands of years ago. Sea level was much lower in those times. Blue holes originally formed like limestone caves did in the last ice age. As the glacial runoff began, the limestone cave system collapsed, eventually resulting in the formation of vertical caves, otherwise called massive sinkholes.

These caves were later flooded with ocean water, thus forming these mighty blue holes. Within the walls of the blue holes, the water remains still, without any currents.  Their dark blue appearance is due to the greater absorption of sunlight, which increases with the increasing depth.

Apart from being a popular dive site and a site for archeological research, blue holes have also been identified as hotspots for microbial life. The microbes found here appear to feed on hydrogen sulfide, which is toxic to other lifeforms. Along with several other blue holes in the world, one of the blue holes in the Bahamas is known to have a mat of microbes completely covering the walls.