Steve Ratcliffe posted a photo of the strange growth he found to Reddit – where experts had a lively argument about what he had found.

He wrote: “I replaced a toilet because it wasn't flushing properly. now I know why ....but what the hell it is I've no idea!!! alien growth right under their bums”.

Steve explained that the customer, who lived on a cattle farm, wasn’t connected to the mains but instead got all their water from an underwater pump via a borehole.

He added that the client had suffered numerous leaks and other plumbing problems over the past few years. Steve initially put that down to the system's age and the fact that it is not connected to the sewage system.

But then he disconnected the U-bend and found… whatever it was.

The bizarre fungus – which looked like something out of a science fiction film – was variously described as a fungus, a biofilm, and an alien.

"The customer has had years of intermittent soakaway issues and backfilling problems...probably due to the age of the system, as it's not on mains drainage," Steve said.

"The toilet age is unknown, but going off the overall state of the house, I guess it's at least 10 years old," he added.

The toilet was almost blocked by a creepy brown-coloured growth that gave some Reddit users nightmares.

One speculated that the grisly growth could have been the remains of a wax seal, while others guessed that it was a biofilm – a sort of colony of bacteria that can be found in all sorts of niched, including on our teeth and even in heart valves.

Angel Medina Vaya, a professor of applied mycology at the University of Cranfield, told Newsweek that while it was difficult to say conclusively what the “alien” might be just from a picture, his best guess was that it was some sort of fungus.

"Fungi are really happy in environments where there is organic matter and high humidity. Hence a toilet that has not been flushed for a long time could be the best place”.