Tunnel valleys are large, elongate and irregular depressions cut beneath the margin of former ice sheets. They are generally believed to play a substantial role for the entire hydraulic system beneath ice sheets and thus also for ice sheet behaviour. Their origin, however, remains poorly understood. Examination of newly collected Danish hydrogeophysical and lithological data has revealed numerous systems of buried valleys 50-350 m deep and 0.5-4 km wide. These valleys are similar to the open tunnel valleys found in the present-day landscape. Infills comprise a variety of sediments often with subglacial clay till and meltwater deposits. The incised substratum consists of limestone, heavy Paleogene clay, sand and silt. The tunnel valleys have preferred orientations and can be divided into several generations, each ascribed individual ice advances or ice sheets occurring during multiple glaciations. The processes forming the valleys appear to prefer pre-existing (open and buried) valleys for the renewed erosion. Thus, old subglacial erosion pathways have been re-used several times, and some of the present tunnel valleys may have been established in the middle or early Quaternary or even earlier. This valley-re-use effect causes apparently anastomosed valley systems to emerge in the data images. The valleys were mainly eroded by meltwater supposed to have drained from subglacially stored reservoirs, probably behind a frozen margin. The water was most likely released in repeated jökulhlaups and flowed in relatively small channels on the floors of the tunnel valleys, which gradually became ice-filled with the lowering of the bed. Selective linear, glacial erosion is also believed to have contributed to the erosion. This, however, was most pronounced for the widest valleys and is supposed to have played a secondary role.