The film claims to have some basis in truth because new micro-ecologies have been discovered in deep caves, especially in Romania. According to film consultant Dr. Christi Lascu (acclaimed speleologist and Romanian editor of National Geographic magazine), 35 brand new species were found in the Movila Caves in the late-1980s. Among the species found was a 10cm centipede with a poisonous bite. There was nothing the size of the creature in the movie, but in theory there is not a limit for the size of animals living down there.
The plot of the movie -explorers in a cave being chased by horrific creatures who were once human- is most probably inspired by the short story The Beast in the Cave (written in 1905, published 1916), one of the earliest ones by American horror writer H.P. Lovecraft, who later would create works as The Call of Cthulhu and At the Mountains of Madness.