"Do you hear that Piero?" Marco says. They wait, silent for a moment. "Dig! Dig! Dig!" they shout, scrabbling desperately at the mud after hearing from far below the faint sound of running water that they crave so desperately. Danger lurks everywhere in the dark narrow tunnels, but the lure of exploration and the competition with other speleologists to be the first to reach the Timavo pulls them ever downward.

This mysterious underground river follows an unknown course of over 37 miles through Slovenia and Italy. It's underground waters were first discovered by Frederico Lindner who began his exploration in the 1830s in search of a much needed water supply for the nearby city of Trieste, which at the time was the main commercial port for the Austrian Empire. In doing so he found the deepest cave in the world, 980 feet deep, but sadly died shortly after his discovery. His search began the tradition that still survives today.

Marco's team are the successors to Lindner, trying to find another way down to the Timavo and help plot its long winding underground course. But they're not the only ones. Just down the road the oldest speleologist group in the world is also working hard in search of the Timavo. Between the two groups a race has started to see who will reach the river first. "Who'll get there first? Us off course!" Marco shouts from 190 feet down.

But both teams are taking big risks as they dig ever deeper. Carbon monoxide gas, narrow dark conditions and falling rocks create a sometimes perilous environment. Something that Marco learned first hand. "A rock broke away at the entrance to the well when I was 165 feet down and I took the torpedo in my side. I made it out, but my kidney had been hit straight on, there was huge internal haemorrageing."

After ten years of digging they are within touching distance of the Timavo, but encounter a new and crushing blow. There is so little oxygen in the air they can't breath. Luckily, Marco has an ingenious idea to pump air down to them and save the expedition. In the final stages they descend, squeezing through a narrow channel they have dug out of the dirt. It scarcely looks big enough, but they crawl through and emerge into a cave.

It's here that they finally find proof of the Timavo. The beetle that Lindner found in his cave is here also and provides the biological proof that right below them is the river. They've won the race: now the really deadly digging must begin...

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