The Quercy causses

The Lot is the best cave-diving ground in Europe and the reason is geology. The causses — Gramat, Martel, Limogne — are plateaus of Jurassic limestone a few hundred metres thick. Rain falls on them and disappears: there are almost no surface streams on the causse itself. The water travels underground, sometimes for tens of kilometres, and comes back out at the foot of the plateau where the limestone meets the river valleys. Those exits are the résurgences, and they are what people dive.

Because the water comes out under pressure at the bottom of a cliff, the entrances are characteristically a pool at the edge of a road or a field, with the passage going in horizontally and down. There is no dry cave to walk through first. You park, kit up, and the dive starts at the surface of the pool.

Four systems account for most of the diving in the region:

• Système de l'Ouysse — the Ouysse sinks on the Causse de Gramat and resurges below Rocamadour at Cabouy and Saint-Sauveur, with Font del Truffe in the same basin.
• Système de Padirac — the underground river of the Gouffre de Padirac, whose outlet at Fontaine Saint-Georges was proved by a fluorescein trace in 1947: 75 kg poured into the Lac des Grands Gours took 114 days to appear.
• Réseau Gramat-Sud — drains 330 km² of the southern Causse de Gramat into the Célé at the Ressel and Marche-Pied.
• Système du Charlan — the Charlan sink at Vidaillac, coming out at the Trou Madame.

Three more sites — Landenouse, Cunhac and Combe Nègre — are independent resurgences with no named system behind them.

The Lot is also where the technique was worked out. French cave divers pioneered the scooter, the staged-bottle push and the rebreather traverse here; the Ressel in particular has been a proving ground since the 1980s and has been pushed past 4 km.