Nidlenloch, Limestone cave in Oberdorf, Solothurn, Switzerland

Nidlenloch is a limestone cave system in the Weissenstein mountain with more than 7 kilometers of interconnected passages that descend over 400 meters below the surface. The network consists of horizontal galleries and older passages shaped by flowing water over geological time.

The cave was first documented in 1828 and became recognized in 1909 as one of the deepest known caves at that time. This discovery made it an important location for cave exploration and scientific study in Europe.

The cave lies in the municipality of Oberdorf and is a protected nature reserve. Access is arranged through the Gasthof Hinter Weissenstein, where visitors leave a deposit and pay a small entrance fee for the key to the locked entrance grille. Around 3,000 people visit each year, and guided tours are also offered.

The entrance at 1,274 m on the northwest flank of the Weissenstein is closed by a solid grille; the key is available against a deposit and a small entrance fee at the Gasthof Hinter Weissenstein. The cave is usually open Wednesday to Sunday, and guided tours are also offered.

In 1909 the Nidlenloch was considered the deepest cave in the world, and it long remained the deepest of the Swiss Jura. It is largely fossil and inactive, with only small trickles in places. The engineer Moser drew a plan and cross-section of the then-known parts in 1868 at a scale of 1:1,000.