Information about the Poltva river from Wikipedia:
"Poltva originates from many natural sources in the area of the park " Pohulyanka ", where the catacombs, which stretch through the whole city to the down town. Tourists, walking the main avenue and admiring the beauty of the Opera House, don't even guess (if only someone will tell them) that just under their feet full-fledged invisible river is flowing - local "Styx". Hidden under a multi-meter layer of "granite sky", Poltva takes away all the products of the life of a modern city and along with the Zakhidnuy Buh river through Poland flows into the Baltic Sea. Sometimes people in Lviv are joking that this is their greatest modern contribution to the European community. "