Okno is a picturesque siphon spring of Nerajčiča – the first major contributory to the river Lahinja. Nerajčiča is only about 1400 meters long and the height difference from spring to its joining with Lahinja is only about 8 meters. The inundation plain is about 50 – 70 meters wide. The area around the spring contains a small wetland.

The term Okno (eng: window) most likely comes from the small walled section of the spring. A small portion was walled off and dammed with only a small window left open where water flowed outward. The inner section of the wall was for gathering drinking water, the outer area was left for cattle and washing clothes. This was one of the ways water was kept clean and healthy. The walling off a small section of spring was a common practice in Bela krajina. Above the spring a small statue of Lady of Lourdes is located. Okno and the entire Nerajčiča river are designated protected areas on a national level. The area around the spring is maintained by the local population.