Krupaj spring is located in the southeastern part of the Krepoljin - Krupaja basin, below the western limestone section of Beljanica, in the area of Milanovac. It is 35 km away from Žagubica. The road that merges Homolje with Resava passes by the spring. Its altitude is 230 m.
The waters of this spring arise with great force from the cave opening which they stream into through the underground canals of Beljanica. Krupaj spring is one of the biggest and strongest karst springs in Serbia. The spring arouse from the cave opening in form of a river flow until 1946, but then a dam about 40 m downstream. In that way, the physiognomy of the spring was changed, thus it started resembling a smaller lake, while over the dam the water flows in the form of a waterfall. The amount of the water depends on the amount of atmospheric sediment that is being excreted, as well as the snow break. A canal emerges from the lake, built on the cliff. That canal used to lead the water to the mills and hydroelectric power plants but now it leads it to the trout pond. The origin of this spring is conditioned by the tectonic structure of this part of Beljanica. Additionally, it was conditioned by the fluvial phase that the Mlava has undergone and the karst processes and evolution of the Krepoljin - Krupaja Neogene lake. The water of Krupaj spring is crystal clear and clean, thus it is drinkable. When they first measured the spring’s depth, it was 7 m. This depth was reached by Aleksandar Milosavljević Ceca in 1995, and later in 1998, he came back with a group of speleo divers from Belgrade. By the means of later explorations in 2007, diver Saša Milanović reached a depth of 105 m. On the 24th of May in 2009, the divers Mirko Bevenja and Branislav Pavlović, from the diving club “Triton”, reached the depth of 123 m which is so far the deepest point of the spring. Due to the strong currents, it was impossible to dive deeper and reach the bottom of the siphon channel which the water arises from. Based on that, it can be concluded that Krupaj spring is much deeper. Krupaj spring surrounded by greenery represents a pleasant ambience for all visitors. That is why the spring represents a real tourist attraction in Homolje. In the vicinity of the spring, one can also find a thermal source which increases the interest of visitors. Krupaj spring is being visited by both domestic and foreign tourists. Taking this fact into consideration, it needs to be invested more in its preservation and enhancement. This will result in an increased number of tourists. For the first time in 1979, Krupaj spring was put under state protection as a natural hydrological monument. In 1995, it was declared a natural monument by a decree of the Government of the Republic of Serbia. Besides the tourism potential, this spring has other potentials as well. One of them is the construction of the water supply network for the surrounding villages (Sige, Milanovac, and Krupaja). The project for this idea is already prepared, and soon its implementation will begin.