What Is a Cenote? A Ring of Them Marks the Dinosaur Crater

Rain that falls on the northern Yucatan Peninsula has almost nowhere to run. The land is flat limestone, and a 2011 review of its aquifer in Hydrogeology Journal calls surface runoff there “practically non-existent”, so the rain sinks and the region lives on groundwater alone.

The holes where that water reaches daylight are cenotes. One 250-kilometre-long arc of them traces the rim of a crater buried under a kilometre of younger rock. The impact that made it ended the age of the dinosaurs.

What is a cenote?

A cenote is a natural sinkhole in limestone, formed when the ground over an underground cavity collapses and exposes the groundwater beneath. The word comes from Yucatec Maya ts’onot by way of Spanish, and it first named the thousands of sinkholes scattered across Mexico’s Yucatan Peninsula, where they supplied the Maya with fresh water.

Slightly acidic rain does the digging

Rain is slightly acidic, so it dissolves the limestone as it seeps down and hollows out caves; NASA’s Earth Observatory credits the same process with the world’s longest underground river. The US Geological Survey puts the pace of limestone dissolution on the order of millimetres per thousand years.

A cavity grows until the ground over it collapses into a sinkhole, a natural version of the collapse that swallowed 300 metres of a Paris street in 1774, where quarrymen had cut the hollow. On most of the Yucatan’s coastal plains the water table sits only 10 to 20 metres below ground, so a collapse there can open straight onto water. Cenotes range from caves filled with groundwater to open pools.

The fresh water floats on seawater

Seawater reaches tens of kilometres inland through the porous rock, and the rain that soaks in floats on top of it. Fresh water is less dense than salt water, the difference that lets ice ride higher in the sea than in a glass.

This floating freshwater lens ranges from under 10 metres to about 100 metres thick. Under Merida, capital of Yucatan state, it is only 60 metres, and the review says septic tanks have made the top 20 metres of that unfit to drink.

The lens stays thin because the water table on the coastal plains sits barely above sea level. By the Ghyben-Herzberg rule of coastal hydrology, fresh water reaches about 40 times as far below sea level as the water table stands above it, so a water table 1 metre up means fresh water to about 40 metres down.

The boundary between the layers is the halocline, and the caves tend to follow it. Even when fresh water and seawater each carry all the calcite they can, a mix of the two can carry more, so the mixing zone keeps dissolving rock. In Quintana Roo’s flooded caves, the review reports, that puts the passages 10 to 15 metres deep near the coast and deeper inland.

A ring of sinkholes traces the buried crater

In 1990 Kevin Pope of Geo Eco Arc Research was studying surface water and Maya archaeology with Adriana Ocampo and Charles Duller of NASA. In Landsat images the team saw “this perfect semi-circular structure”, as Pope later told NASA’s Jet Propulsion Laboratory. It was the arc across the peninsula’s northern tip now known as the Ring of Cenotes.

They turned to gravity and magnetic data collected by Pemex, Mexico’s state oil company, while it searched for oil. The data showed a large circular structure that Glen Penfield, a geophysicist on a 1978 Pemex survey, had already read as an impact crater, though Pemex’s own geologists had it down as volcanic rock. Pope’s team reasoned that the cenotes marked fractures in the structure’s buried rim, and they were among the first to link the crater, now called Chicxulub, to the mass extinctions at the end of the Cretaceous.

In 1995 Hildebrand and four colleagues used gravity data to put the crater’s diameter at about 180 kilometres, a size the cenotes confirmed. The match, they wrote, “suggests that cenote formation is closely related to the presence of slump faults near the crater rim.”

The asteroid story is half right

The popular shorthand says the asteroid that killed the dinosaurs created the cenotes. The impact decided where the ring’s holes would line up, but it did not dig them.

When it struck, 66 million years ago, the Yucatan lay under a shallow sea. The crater became a basin that filled with sediment. NASA’s Photojournal says scientists believe the impact altered the rock beneath, so that the limestone laid down later, which erodes very easily, wore away more near the crater rim than elsewhere.

On that account, the erosion made the ring of sinkholes and a trough only 3 to 5 metres deep and about 5 kilometres wide, so subtle that on foot you “probably would not notice it”. The holes in the ring are younger than the impact that lined them up.

The crater from the impact that ended the age of the dinosaurs is about 180 kilometres wide, yet at the surface its clearest mark is a trough you would probably walk across without noticing.

Cenotes also turn up far from the ring. The review cites a 1996 map of more than 7,000 cenotes and other water holes, and notes that cenotes cluster on the coastal plains, where intruding seawater may speed the dissolving.

The Sacred Cenote’s dead were mostly children

The popular story says the Maya threw young maidens into the Sacred Cenote at Chichen Itza, whose name means the city at the edge of the well of the Itza. The first description of the site is in notes that Diego de Landa, Bishop of Yucatan, probably wrote in 1566. In the translation the Penn Museum printed in 1911, the notes record “the custom to throw into it live men as a sacrifice to the Gods in time of drought”.

A 2024 study in Nature says the maiden idea was popularized by “sixteenth century Spanish colonial accounts and early twentieth century investigations following the dredging of the Sacred Cenote”. That dredging began in 1904 under Edward H. Thompson, the US consul at Progreso.

The bones do not support the maiden story. “More recent osteological analyses indicate that the bodies of both males and females were deposited in the Sacred Cenote,” the study says. Citing earlier work, it puts the cenote’s dead at more than 200 ritually sacrificed individuals, mostly children.

The Maya drew water from the ring of sinkholes for centuries, long before anyone knew it traced the rim of a crater 180 kilometres wide. Strictly FYI.