Mont Terri, Mountain summit in the Jura, Cornol, Switzerland

Mont Terri is an 804-metre summit in the Jura near Cornol, an anticline whose Jurassic layers dip 42° to the south-southeast. Its north flank carries the 4,068-metre Mont Terri motorway tunnel, opened in 1998, and a security gallery from which the Mont Terri Rock Laboratory was excavated in phases from 1996 to 2018.

The summit plateau was fortified from the late Bronze Age, around 1250 BC, and occupied by a Celtic settlement destroyed before a dry-stone murus gallicus rampart was built, possibly linked to the Helvetii and Rauraci return in 58 BC. A Roman garrison stayed in the mid-4th century, and a Carolingian tower stood on the highest point.

The name Mont Terri likely comes from the Franche-Comté patois mont Tairi, said to mean "mountain whose springs run dry". Before the Napoleonic era the peak was called Mont Terrible, a fancy that named the French département du Mont-Terrible, which existed from 1793 to 1800. Michel Bussi set part of his 2012 novel Un avion sans elle here, and Jean-Claude Pirotte evoked the mountain in his 2012 poetry collection Ajoie.

The summit plateau, about 4 hectares, is reached on foot; the A16 motorway crosses the massif through the 4,068 m Mont Terri tunnel, opened in 1998, which links Saint-Ursanne and Courgenay. The rock laboratory is a working research site entered from the tunnel's security gallery, not a public attraction, so it cannot be visited on a hike.

The rock laboratory was built from the security gallery of the Mont Terri motorway tunnel, which was excavated first as a reconnaissance gallery between 1988 and 1998. Research began in 1996 with small niches cut into that gallery, followed by new galleries in 1998, 2008 and 2018. The clay studied is the Opalinus Clay, a 130 m thick layer deposited 174 million years ago.