THE Dead Sea is, as its name implies, far too salty to be of use to fishermen or farmers. But its mineral-rich waters are valued by the owners of the spas that thrive along its shores in Israel, Jordan and the Palestinian territory of the West Bank. The spa industry, however, faces a threat from a plague of sinkholes that have struck in recent years. These have damaged roads and buildings at Ein Gedi beach, in Israel, and hit the Mineral Beach Spa in Mitzpe Shalem, an Israeli settlement in the West Bank, so hard that it is unusable.

Until now, it has been impossible to predict more than a few weeks in advance where a sinkhole will appear. But, as he reports in Geology, Meir Abelson of the Geological Survey of Israel thinks he can change that. Employing buried monitoring devices, he believes he can forecast where such holes will form several years before they actually do so.

Most of the more than 6,000 sinkholes that have struck the west coast of the Dead Sea recently are the result of that sea being starved of water as the rivers flowing into it have their contents extracted for human use. This has caused the sea’s level to fall and the pressure it exerts to diminish. That drop in pressure is felt in the surrounding rocks, which are often porous and water-filled. The rocks are a battleground between salt water from the lake and fresher water from elsewhere—a battle that, because of the pressure drop, the fresher water is winning.

When this incoming water makes contact with the extensive underground rock-salt formations that surround the Dead Sea, it dissolves them. That creates cavities which, if they grow too large, eventually fail to support the rock layers above. The consequence is the sudden collapse of those layers into a hole in the ground, taking anything on the surface with them.

A technique called interferometric synthetic-aperture radar can give a few weeks’ notice of such a collapse. It permits researchers to spot subtle signs of subsidence that often presage a sinkhole’s formation. This grants enough time to evacuate the site, but not enough to plan the relocation of an extensive spa facility. Dr Abelson, though, speculated that the disintegration of the rock-salt formations which creates the holes might cause minor tremors well in advance of a collapse.

To determine whether this was so, he and a team of colleagues drilled, in 2012, five small boreholes around Mineral Beach. They then placed sensors into each borehole, at depths ranging from 11 to 25 metres. These sensors, called geophones, recorded movements of the earth and transmitted their data regularly to a nearby monitoring station.

During the two months in which they operated Dr Abelson’s geophones detected 82 seismic events, of which 75 came from the surface layers rather than being a result of the movement, deeper underground, of tectonic plates. Crucially, he and his colleagues were able to use the slightly staggered arrival times at different geophones of these tiny tremors to locate each tremor’s epicentre. This let them make a map of Mineral Beach that showed where the crust was beginning to buckle because of cavity formation below. They then waited and watched, to see if the epicentres were at places where sinkholes subsequently formed.

The first such collapse was in 2014, a little more than two years after the team had detected a series of small tremors emanating from that very spot. Seven additional sinkholes formed in 2016. Again, they were in places where the researchers had detected clusters of tremors during their two months of monitoring. Not all of the tremors they recorded have yet been followed by the formation of sinkholes. But, crucially, all the sinkholes that have formed so far have been at the epicentres of tremors.

That observation argues for the installation of permanent geophone networks in tourist areas around the Dead Sea. Indeed, tracking changes in the rate and strength of tremors, before any distortion of the ground detectable by radar shows up, may provide information about when a sinkhole might be expected to form, as well as where. That would give building owners more notice of problems, permitting them to move facilities in good time.

Nor is the Dead Sea the only place where Dr Abelson’s system has the potential to do good. Several states in America, including Florida and Missouri, have large swathes of land sitting on top of rock salt and other materials that easily wash away and generate sinkholes as a result. Indeed, southern Missouri has seen over a hundred sinkhole collapses since 2007, and would benefit greatly from having means in place to detect when and where they are likely to happen.