Creatures resembling dragons dwell deep within the caves of Slovenia. And now with the help of biologists, a new brood is set to hatch at an unusual aquarium, accessible by underground train.

This crystal clear egg is one of three recently laid by an olm, a cave amphibian whose long sinuous body, stubby legs and frilly gills led people in the 15th century to believe it was the offspring of dragons.

Though it does not breathe fire, the olm has several attributes fitting a creature of mythology. It can live as long as a hundred years and survive without food for 10. It is blind, but hunts using its incredible sense of hearing and smell, and it can detect electric and magnetic fields.

Female olms only reproduce once every six years and it takes them until the age of 15 to become sexually mature. The aquarium, located at a tourist site known as Postojna Cave, last expected baby olms in 2013. But none of those eggs hatched and several were eaten by other olms in the tank, explained Sašo Weldt, a biologist who works there.