The laying of eggs by the blind amphibian species of caves in Slovenia has been witnessed only a few times, prompting cave operator Postojnska jama to call the latest case a miracle of nature.

The in-house biologists are waiting impatiently to see if the pregnant female will lay more eggs. The first time an olm was observed laying eggs in the Postojna cave in 2013, she produced 22.

One of the guides in the very popular cave noticed an egg sticking to the wall of an aquarium on Saturday. The cave's biologists, Primož Gnezda and Sašo Weldt, were immediately called in.

They observed that the egg was being protected by a pregnant female, who attacked any of the other six olms if they got too close, which is why they moved the others to another aquarium.

The biologists are now waiting to see whether the female will lay more eggs. It is also possible that her body will re-absorb them in order to save energy for the next reproduction, Postojnska jama also said in a press release.