This entry is part 1 of 3 in the series Blue Holes Theme Week

Blue Holes & Cenotes

Typically, the matter beneath our feet is a matter of no concern.

It’s literally rock solid, and we move about our lives without paying it much attention.

Not all Earth is equal, though. Some soils make fantastic bedrock, while others are subject to dissolution and erosion.

Sometimes, when chemical processes or erosion work on rock, the ground can suddenly disappear. These happenings can be rather dramatic or massive. We call the result a sinkhole.

Though sinkholes can form in a number of soluble rocks, such as salt beds and gypsum, they happen most frequently in limestone.

Limestone is a sedimentary rock formed when calcium carbonate precipitates out of water. This process occurs frequently at coral reef sites, where organic compounds produce the carbonates over millions of years. Limestone is particularly susceptible to dissolution. Many continental spots were, at some point, the bottoms of oceans, so limestone deposits commonly sit in places we do not currently associate with reefs. Mammoth Cave in Kentucky, for example, is composed mostly of limestone. Over the eons, the rock has dissolved into the world’s largest cave system.

It follows that places rife with limestone develop many sinkholes. Florida is basically a limestone bed, so, unsurprisingly, sinkholes pock the landscape.

In a few circumstances, special types of sinkholes exist, including a couple that partner with water in a manner beyond dissolution.

During the ice ages, the seas were much lower than current levels. When the ice melted and oceans rose, some of the sinkholes formed during this period were filled with seawater.

We call these instances blue holes.

Though blue holes often precipitate in oceans, they can happen on islands or inland.

These gorgeous marine caves feature striking color gradients and can be quite deep. The planet’s deepest – Taam Ja’ Blue Hole – goes down 1,380 feet (420 meters)! The largest by volume – the Great Blue Hole, pictured above – sits off the coast of Belize; its diameter is an incredible 1,000 feet (300 meters)!

Blue holes, unsurprisingly, attract a bevy of diving attention, including from those looking to probe depth records.

Blue holes are often large and circular. Sometimes these types of sinkholes form on the mainland, far from rising sea levels, but occasionally they manage to strike water anyway.

Particularly prominent on the Yucatan peninsula are a sinkhole version called cenotes.

This name comes from a Yucatec Maya word for accessible groundwater: ts’ono’ot. True to their name, cenotes are sinkholes that expose groundwater.

The Yucatan has more than an estimated 10,000 cenotes. Originally, geologists applied the term only to Mexican sinkholes but have since applied the moniker to other locations, such as Greece, Cuba, Australia, and the United States.

Cenotes vary wildly in size. Many are tiny, with scant access to water. The largest, however, connect to gargantuan caves thanks to the inherent hydrological systems. In the Yucatan, divers have mapped cenote systems to lengths over 230 miles.

Surface water on the Yucatan is scant, with nearly no river activity and a dearth of lakes. The ancient Maya utilized cenotes for survival, basing their civilizations around them. Chichen Itza, the famous pre-Columbian city, sat near sinkholes, including the Sacred Cenote, which also served as a ceremonial center. The Maya believed cenotes were a doorway to the afterlife and a dwelling for their rain god, Chaac.

Archaeologists have discovered artifacts and human remains in cenotes, evidence of the role they played in the culture.

Intriguingly, the preponderance of cenotes on the Yucatan helped uncover the potential cause of the demise of dinosaurs.

Geologists noticed a strange layout of cenotes near the ocean, forming an arc, as if a full circle were interrupted by the ocean.

Further investigation yielded a gravitational anomaly in the vicinity. Though it was difficult to see with the naked eye, the cenote arrangement hinted at a geological oddity. The cenotes likely would have formed rather uniformly under normal circumstances, but something had caused a bizarre pattern.

Scientists finally realized they were looking at part of an astounding impact site, the remnants of the Chicxulub meteor.

This space rock likely wiped out the dinosaurs, and it left behind a semi-ring of sinkholes as evidence of its existence, yet another example of the universe’s sublime talent for mixing the awesome with the destructive.