The karstic system of Han-sur-Lesse is the archetype of underground meander cutoff. Located in the “Calestienne”, a narrow bench extending along the northern margin of the Lower Devonian Ardenne, it comprises all caves and underground flow paths related to the epigenetic Lesse River across the Givetian limestone anticline of Boine. The Han-sur-Lesse caves result from a two-stage karstogenesis. First, ghost-rock karstification, the residual alterite of which is still observed locally in the system, affected the substratum of a fairly uniform topography under hot and wet climates, probably during the Cretaceous and the Paleocene. Second, as a result of the Ardenne uplift during the Neogene, river incision created a hydraulic potential that led to removal of the alterite by underground water circulation. Currently, the underground circulation short-circuits a meander of the Lesse, from the swallow hole of “Gouffre de Belvaux” to the resurgence at “Trou de Han”. Another small underground stream is separated from the main circulation by less permeable nodular shale and argillaceous limestone strata. The overall system is essentially made of horizontal galleries, with large rooms located at structural nodes or produced by the coalescence of superposed galleries. The river accumulated pebbles, sand, and clay in the caves, often capped by flowstones, the whole recording Quaternary climate oscillations.