Marble Arch Caves: the science behind the tracing of water
A green, glowing river might look weirdly spectacular but for Paul Wilson it is part of daily life.
As a hydrogeologist with the Geological Survey of Northern Ireland, it is his responsibility to check where water comes from and goes to and what effect it is having on the land as it travels along.
That's because some of the ground in Northern Ireland is of a particular kind.
"We call this karst," Paul said.
"It's a unique form, where the rocks dissolve out because of the slight acidity of the rainwater and then it creates these cavities in the ground, which means streams can actually sink underground then reappear again in really dramatic fashion sometimes."
This is what has happened over the millennia in County Fermanagh at the site of the Marble Arch Caves.
Three rivers - the Owenbrean, Aghinrawn and Shru Croppa - have gradually made their way underground and carved out the incredible chambers we can see today.
But the caves are still part of the underground journey of the water. It emerges further down the mountains and because people will always need water sources, tracking that is important.
"We need to understand where water comes from and where it goes to," said Paul.
"One of the ways in which we do that is by doing tracer tests. It's not that complicated - we insert dye and it comes out somewhere else.
"But we need to know that, to know where the water is coming from and going to, so as to be able to manage it better."
As part of the Northern Ireland Science Festival, people got the chance to try out being a hydrogeologist for themselves, where the Shru Croppa river makes its way through cracks in the rocks to join the Owenbrean and Aghinrawn in the caves.
A dribble of dye is all it takes for the torrent to become an alien-green flow, much to the delight of the onlookers.
"It just went into a little slit over there and it was really awesome to see," said Cian Doyle, who came along with his mother.
"I wanted to get out to see it because there's no point in sitting round playing the PS4 all day."
The green dye - it also comes in pink - is completely harmless. But it lets hydrogeologists trace the flow so they can predict the water's effects above and below ground.
Testing the way the water behaves tells the scientists all sorts of things, like how fast it is moving, its volume, and what it is doing to the rocks around it.
In a place like the show caves, that sort of information is invaluable.
"In times of heavy rain, the water underground can, in parts of the cave, actually reach the cave roof," said Helen Carleton, a duty officer at the Marble Arch Caves.
"So it's very important for the duty officers to be able to monitor the water throughout the day to be able to know when to close off parts of the cave to make it safe for the public."
Once the dye is in the water, it takes about 24 hours to make its way underground, dissipating as it goes. But that's nothing compared to the millennia it can take to carve out these chambers.
"With water you can see differences in hours and we saw it today," said Paul.
"We were at Shru Croppa twice and the water level at the start of the day was higher than it was at the end of the day and the way the dye went into the water was completely different.
"So that's why I enjoy working with water because you can see differences in a very short period of time with the way water changes each day."
But rocks? Well, that's more of a waiting game…