Surveying the underworld

​(From "Horizons" no. 104, March 2015)

There’s jungle everywhere. Here in the Mexican state of Quintana Roo on the north-eastern Yucatán Peninsula, the climate is tropical – hot and humid. Yucatán extends from the mainland into the Caribbean like a giant horn. But despite the rainforests and the mangroves, when you drive the 300 miles from Cancún on the outermost edge of Yucatán towards the border between Mexico and Belize, you don’t cross a single bridge. This is because the region is completely without rivers or streams – or, to be more precise, without visible rivers: water here flows under the ground.

The bedrock is traversed by a huge network of caves. There are hundreds of rock cavities, channels and tunnels, most of them filled with water. Above ground, the only testament to this hidden labyrinth is the so-called cenotes – sinkholes created when the roofs of caves collapse. There are more than 3,000 of these natural cisterns in Yucatán. This area was once home to major centres of the Mayan civilisation, and the Mayans believed that the cenotes were gateways to the underworld.

One of those Mayan centres was the city of Tulum. Here we find the second-longest and fourth-longest caves in the world. “They are probably even linked to each other, which would then make them the biggest cave system in the world”, says Philippe Renard, a hydrogeologist at the University of Neuchâtel. Together with colleagues from the Federal Institute of Geology in Austria, he’s investigating one of these cave systems: the so-called Sistema Ox Bel Ha, which is some 160 miles long. The main goal of this research project is to develop mathematical models to explain how such cave labyrinths are formed and how they function.

Dissolving limestone

The prerequisite for porous bedrock is the rock itself: the Yucatán Peninsula is a giant limestone slab that was created over millions of years when the region was still under the sea. Dead coral accumulated and transformed into a plate of limestone over a mile thick. Limestone disintegrates relatively quickly when it comes into contact with carbonic acid dissolved in water, and the result here was a series of clefts, pores and cavities.

Today, the caves are so long that they are filled with sea water for dozens of miles inland. “In areas near the coast, such as in Tulum, a mixture of salt water and fresh water causes the rock to dissolve over the course of millennia”, says Renard. Because saltwater has a lower specific gravity than its fresh counterpart, it forms the bottom layer when the two types of water come together. Rainwater gradually trickles through the porous, chalky soil and flows onto the salt water. The two water types don’t completely mix, instead forming an intermediate layer. “We believe that more limestone has dissolved in those places where this layer occurs”, says Renard.

Renard and his team are using computational models to investigate how the cave system was formed, but by no means are mathematical abilities all that’s needed here. The researchers are dependent on data to feed into their models to test their plausibility, because karst systems are complex. There is a whole series of questions to be answered in trying to determine the extent and the speed of limestone disintegration: What is the precise rock composition? What is the composition of the water, and how do rock and water interact? And, lastly, what substances and sediments are transported through the pores and gaps in the rock?

First, of course, the scientists have to know where the caves actually are, and how big they are. Because to no extent have all of them been mapped. “Our Austrian colleagues track down unknown caves by means of electromagnetic measurements made from a helicopter”, explains Renard.

The helicopter flies above the jungles of the karst terrain and sends out electromagnetic waves into the ground. As these waves travel at different speeds through limestone and water, the researchers can use this method to locate water-filled caves quite precisely. In order to measure the caves, the scientists have developed an underwater device comprising a laser and a camera. Specialist cave divers take these into the grottos and use them to determine their dimensions. “Divers have also helped us to bring sensors into the caves, with which we can determine the rate of the water flow”, says Renard. Other sensors provide information about changes in the water level in the karst system, and other devices collect data about the properties of the karst rock itself. “Of course, we also use information that other researchers have already published”, explains Renard. “In order to determine the long-term development of the karst system, for example, it’s important to know about the fluctuations in the sea level during the last Ice Age”.

Visibility for 250 metres

It’s too early to be able to offer any conclusive results, says Renard. The mathematical models are still being created by his doctoral students Axayacatl Maqueda and Martin Hendrick, and they’re not finished yet. “But we understand the chemical composition of the groundwater in this region better than we did”, he says. The project’s chemical analyses have proven, for example, that the fresh water alone is far more aggressive than at first believed, even when it doesn’t actually mix with salt water. The high level of groundwater, the tropical climate and the intense biological activity in the region all increase the production of carbon dioxide, which in turn leads to a higher rate of limestone disintegration.

Whilst creating models might be pure research, the project may also yield practical applications, such as preventing water contamination. The inhabitants of Yucatán have used the cenotes as freshwater reservoirs since Mayan times, and the water in these chambers is so clear that visibility in it can extend up to 250 metres. However, hotels and other tourist facilities have been proliferating here in recent years, meaning that giant quantities of wastewater could soon flow into the porous karst rock and contaminate the drinking water. It will only be possible to take appropriate protective measures for this unique geological landscape if we know precisely how its cave systems function.

Simon Koechlin is the chief editor of Tierwelt and a science journalist.