The Kras is a region rich in limestone, with caves formed by water erosion. In some places the roof of the caves collapsed, known as Dol or Dolina. The Reka River flows for 55 km as a superficial stream until it reaches the Kras, where it disappears underground for 300 meters in a cave under the village of Skocjan. The roof of the cave collapsed in two places known as Velika Dolina and Mala Dolina. The river continues its flow in the cave for another 3.5 km, with underground lakes and waterfalls. There are several chambers, the largest is the Martelova Dvorana, 308 meters long, 123 meters wide and 146 meters deep. The river continues its flow for another 30 km underground before it reappears.