YouTube graffiti videos track cave water
Scientists are using YouTube videos to study a spectacular cave just outside Riyadh, the capital of Saudi Arabia.
Groundwater fills Dahl Hith, and it has become a popular weekend bathing spot for city residents wanting to cool off in this super-arid region.
And after decades of decline because of abstraction for nearby irrigation, the water levels are now rising.
Researchers have tracked this return by documenting cave-wall graffiti in the online movies posted by bathers.
They use the writing as reference points to estimate the position of the water table.
Some of these scribbles have dates in them; and some of the videos themselves incorporate time information.
All of this data has allowed the team to reconstruct past conditions in Dahl Hith.
"Since smartphones have become so popular and deliver quite good video quality - such data becomes useable. A few years ago, some people would have criticised it as 'grey data', and said 'you shouldn't use it'. But since there is no other data, you need to be creative," said Nils Michelsen, from the Institute for Applied Geosciences, TU Darmstadt, Germany.
He has been presenting his team's work at the European Geosciences Union (EGU) General Assembly in Vienna, Austria.
The cave study is another example in what is becoming a significant trend - the trawling of social media for scientifically valuable information.
Researchers are now "mining" platforms like Twitter for pictures and keywords.
Another application demonstrated here at EGU exploits the tweets of Jakarta residents to track the extent of flooding in the Indonesian capital.
Mr Michelsen concedes such grey data needs to be used with care, but believes the approach can be very powerful.
His examination of almost 40 YouTube videos made in Dahl Hith reveals that water levels have risen by about 40cm a month over the past two years.
Indeed, it is now coming up so fast that some of his graffiti "benchmarks" have been submerged.
The thinking is that the discharge of treated waste water some 10km from the cave is topping up the bathing pool.
Dahl Hith is a fascinating site for geologists. It is one of the few locations where you can see an outcrop of the anhydrite (calcium sulphate) caprock - the seal that traps the Arabian Peninsula's oil.
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos