A very rare 'rat king' has been taken to the University of Tartu Natural History Museum after being discovered in Polva County.

It is likely that it is just the second registered finding of such a phenomenon in the world this century, Estonian media reported on October 22.

A "rat king" is a collection of rats whose tails are intertwined and bound together, either with hair or sticky substances or getting tied together.

The Polva County"rat king" was found in an auxiliary house meant for domestic birds.

"My mother went to feed the birds in the morning, opened the door and the rats were in front of the door as if on a tray. They had burrowed a tunnel right in front of the door and gotten stuck in that tunnel. My mother could not do anything. I tried untying them, but it was quite complicated to understand if their tails were tied or if they were stuck to the underlay," said Johan Uibopuu, one of the finders of the "rat king" to ERR, an Estonian news website.

University of Tartu Natural History Museum curator Andrei Miljutin said a "rat king" can form in nests while rats sleep. If their tails are covered with sticky substances such as blood or sap, the ends of their tails can freeze together.

"And the rats start to move when they wake up and move in different directions, they end up tying their tails together and can no longer get free. The more they struggle and pull, the tighter the know gets," Miljutin said.

13 rats had gotten stuck in this particular situation.

According to Miljutin, this is a very rare finding. There have been some 60 registered cases of "rat kings" found in the last 500 years. Usually, the rats are not alive when researchers get their hands on them, which is why video footage is also very rare.

"The last finding was in 2005 to my knowledge and according to Wikipedia, that was the only finding in the world this century. And now this one is the second this century," the museum curator said.

Interestingly enough, Wikipedia notes that the sighting in 2005 was also found in Estonia.