The BBC's Jonathan Amos reports that one scientist, Nils Michelsen from Germany, used more than 40 YouTube videos of one graffitied cave in Saudi Arabia to monitor the rising water in the cave.

By looking through the videos for particular graffiti spray painted on the walls in the cave (known as Dahl Hith or Ain Heet), Michelsen could monitor the water levels, looking for how much the water rose over certain graffiti marks over time. He figured out that the groundwater in the cave was rising at a rate of about 15 inches every month for the past two years. The most likely source for the extra water is probably treated wastewater from a nearby water treatment plant.

With the water rising so fast, Michelsen is concerned about the stability of the cave. The rocks that make up the cave are limestone and anhydrite, both of which are easily eroded by water. In fact, many limestone caves were formed by water slowly hollowing out the ground. Water rising in the cave could potentially weaken the rocks in the cave, creating a safety hazard. "There are blocks that come down from time to time," he tells BBC News. We actually gave the recommendation to the local ministry that they should close the cave. There are so many people in there, it's somewhat dangerous."

Michelsen's research was presented at the European Geosciences Union (EGU) General Assembly.