The Julian and Karavanken Alps traverse the country. The highest mountain peaks are Nanos, ; Vremščica, ; Snežnik, ; and Triglav, . The principal rivers are Sava, Tržič Bistrica, Kokra, Kamnik Bistrica, Sora, Ljubljanica, Mirna, Krka, and Kolpa, which serves as a boundary with Croatia. The principal lakes are Black Lake (), spreading into seven lakes, of which the highest is over above sea level; Lake Bohinj; Lake Bled, in the middle of which on an island is built a church to the Blessed Virgin, amidst most picturesque scenery; Lake Cerknica, above sea level, varies annually in extent from about . It was known to the Romans as Lacus Lugens or Lugea Palus, and is a natural curiosity. Dante Alighieri mentions it in his Divine Comedy (Inferno, xxxii). The Ljubljana Marshes cover an area of . Hot and mineral springs are found at Dolenjske Toplice, Šmarješke Toplice, and Izlake. There is an interesting cave at Postojna.