There are thousands of tumuli throughout all Croatia, built of stone (Croatian: "gomila", "gromila") in the carst areas (by the Adriatic Sea) or made of earth (Croatian: "humak") in the inland plains and hills. Most of these prehistoric structures were built in the 2nd and 1st millennium BC, from the middle Bronze Age to the end of the Iron Age, by the Illyrians or their direct ancestors in the same place; the Liburnian inhumation of dead under tumuli was certainly inherited from the earlier times, as early as the Copper Age. Smaller tumuli were used as the burial mounds, while bigger (some up to 7 metres high with 60 metres long base) were the cenotaphs (empty tombs) and ritual places.