Constant rain and other weather influences are slowly dissolving the entire range. Each year, it recedes approximately . Some millions of years ago, the mountains reached as far as Stuttgart. In some places, the limestone was more resistant to decay and thus the recession of the main range has left small mountains (called Zeugenberge—witness mountains or outliers; e.g. the Achalm or the Hohenstaufen) which testify to the former territory of the range. The omnipresent caves are great tourist spots, beautiful and not very crowded. Many different types can be found, from dry dripstone caves to caves that can only be entered by boat. Sometimes the discharge of the water from subterranean rivers can be spectacular, too, e.g. the Blautopf, a source for a tributary of the Danube. Also because of the porous limestone, the Danube nearly disappears near Immendingen (in 1911, 1921, 1928 and 1943 it disappeared completely), only to reappear several kilometers further down. Most of the water lost by the Danube resurfaces in the Aachtopf, a spring for a tributary to the Rhine.