Průrva Ploučnice, Underground canal in Noviny pod Ralskem, Czech Republic.

Průrva Ploučnice is an artificial water tunnel cut through Turonian sandstone at Noviny pod Ralskem. The Ploučnice enters a gorge between steep rock walls, turns right after about 50 metres into a 13-metre tunnel, resurfaces for about 10 metres, then passes through a second tunnel 41 metres long. Both were cut by hand with pickaxes, measure 3.5–4 metres high and wide, and have openings in the walls for five wooden weirs; at the end the water drops about a metre into a pool below the rock wall.

The tunnel was cut probably in the 16th century as the outlet of a reservoir meant to drive an iron hammer at Noviny pod Ralskem. The Ploučnice was dammed with an earth embankment and diverted into the gorge called Pekelný jícen, but the Turonian sandstone let water through, so the reservoir never filled and water had to be brought by a millrace from Stráž. The site was declared a cultural monument on 17 December 1997.

The name has several variants: Pekelný jícen (Infernal Trench), Hromová rána and Čertova díra (Devil's Hole). In the 19th century a local miller brought visitors through the gorge by torchlight. Today canoeists paddle the Ploučnice through the tunnels when the water is high enough.

The area is accessible via marked walking paths, and you can view the rock formation and water flow from several vantage points along the tunnel. Wear sturdy shoes as the ground can be uneven in places and the area near the water may be slippery.

The name Průrva means "breakthrough" or "breach", not a natural vanishing of the river. The tunnel was cut by hand with pickaxes and shovels, probably in the 16th century, as the outlet of a reservoir meant to drive an iron hammer at Noviny pod Ralskem. The sandstone let water seep through, so the reservoir never filled.