The mystery of what appears to be a screw fixed inside a rock (pictured) has perplexed scientists . An object that appears to be a screw fixed inside a rock has captured the world's attention since it was found in the 1990s, but the debate rages on about what it really is. Russian researchers believe the unusual object is 300 million years old, leading some people to claim that it may be proof of a highly advanced lost human civilisation, or even the work of aliens. However, experts suggest that there may be a more earthly answer –  that the ‘screw’ is in fact a fossilised sea creature. The researchers stumbled across the ‘screw’ after searching the Kaluga region after an asteroid strike occurred there, Ancient Code reported. The ‘screw’ measures an inch (2cm) long and was collected by a UFO and paranormal research team called the Kosmopoisk Group. They claim that the stone is between 300 and 320 million years old and after X-raying it, believe there is another hidden ‘screw’ inside the stone. But more conventional scientists have not got a look at the object, so have had to base their speculation on grainy pictures posted online. The Kosmopoisk Group has not said what the screw is made of, or given any detailed information about it. The self-styled group, which is also known as Spacesearch and the Russian Scientific Organisation, conducts expeditions to sites that are believed to have extraterrestrial activity or unusual creatures - and has a mixed reputation. Scroll down for video . The screw-like shape may actually be the reversed-shape of the creature, which gradually dissolved while the rock formed around it. Fossils of Jurassic crinoids collected in Israel are pictured . The leading hypothesis from experts who have seen photos of the stone is that the ‘screw' is actually the remains of an ancient sea creature called a crinoid. The screw-like shape may actually be the reversed-shape of the creature, which gradually dissolved while the rock formed around it. Some 600 species of Crinoids are now extinct but the creatures live on in different forms in modern seas. They have five  arms to snatch passing prey and a hidden mouth on the top. It is thought that the fossilised creature in the mysterious rock is a form of ‘sea lily’ – a type of crinoid that grew a stalk when it became an adult, to tether itself to the seabed. Crinoids were extremely common millions of years ago, and countless fossils have been found, some segments of which do resemble screws. Russian researchers have dated the unusual 'screw' to 300 million years old, leading some people to claim that it may be proof of a highly advanced lost human civilisation, or even the work of aliens (stock image) Researchers stumbled across the ‘screw’ after searching the Kaluga region after an asteroid strike occurred there. The ‘screw’ measures an inch (2cm) long and was collected by a UFO and paranormal research team called the Kosmopoisk Group . There are hundreds of species of extinct Crinoids, although the marine animal continues to live in seas today. Crinoids have a mouth on the top surface - next to their gut and anus - that's surrounded by feeding arms to grab passing prey. They generally have five main arms and a stem to attach themselves to rocks or the seabed. In fossilised form, the stem does resemble a screw. It's thought they were more common in ancient times, because limestone beds dating to the mid Palaeozoic are full of Crinoid fossils. The first Crinoids are thought to have appeared in the Ordovician period, between 488 million years ago and ending 444 million years ago and underwent a selective mass extinction at the end of the Permian period, where they evolved more flexible arms. Crinoid fossils look slightly different to reflect the variations across the species. However, some say that the stalks of crinoids were typically much smaller than the ‘screw’, with slightly different markings, and have discarded the theory. More far-fetched theories include that the 'screw' is the remains of an ancient form of technology that proves aliens lived on Earth millions of years ago, or that our ancestors were far more advanced than we think. Nigel Watson, author of the UFO Investigations Manual told MailOnline: ‘Lots of out-of-place artefacts have been reported, such as nails or even tools embedded in ancient stone. 'Some of these reports are…misinterpretations of natural formations. ‘In this case sceptics claim the screw is nothing more than the fossilised remains of a crinoid marine animal, while believers note that the screw is very different from the surrounding fossil evidence. ‘It would be great to think we could find such ancient evidence of a spaceship visiting us so long ago, but we have to consider whether extra-terrestrial spacecraft builders would use screws in the construction of their craft. ‘It also seems that this story is probably a hoax that is being spread by the internet, and reflects our desire to believe that extra-terrestrials have visited us in the past and are still visiting us today in what we now call UFOs.’ The leading hypothesis is that the ‘screw; is actually the remains of an ancient sea creature called a crinoid. It had a thick stem that attached it to the sea bed, which looks a bit like a screw when it's fossilised (pictured) Some 600 species of Crinoids are now extinct but the creatures live on in new forms in our seas today. This image shows a modern Crinoid, known as a Variable bushy feather star in the Philippines . The Alien that terrified people in the 1979 sci-fi film, was inspired by a crinoid. Swiss surrealist artist, Hans Rudolf Giger, created many artworks based on fossils, which were seen by Ridley Scott who commissioned Giger to make props for his film. Mr Giger's designs - one of which was all too memorably seen bursting out of a character's chest on screen - went on to win his team an Academy Award for Best Achievement for Visual Effects on the 1979 film and the franchise that followed. The Alien that terrified people in the 1979 sci-fi film, Alien (pictured) was inspired by a Crinoid . He was inspired by a 300-million-year-old Crinoid fossil, which was stolen in 2012. While the piece was returned to the Aathal dinosaur museum in Zurich, Switzerland, from which it was taken, it was slightly damaged. Museum curator Dr. Thomas Bolliger said: 'It was comparatively undamaged - although one of the arms had broken off. 'It really is a remarkable fossil and shows two separate species of a spineless animal that lived at the bottom of the sea around 300 million years ago.' A museum spokesman added: 'It is a very valuable but also a very unique fossil and it will be very difficult for anybody to sell or indeed to keep secret given the publicity around the theft.' Alien abduction: Swiss surrealist artist, Hans Rudolf Giger, created many artworks based on fossils, which were seen by Ridley Scott who commissioned Giger to make props for his film. This rare Crinoid fossil was stolen from a museum and later returned damaged .
'Screw' was found in a stone by UFO researchers in Kaluga region, Russia .
The unusual object is said to be 300 million years old .
Some  think it's proof aliens used to live on Earth or a radical civilisation .
But it's more likely to be a fossilised sea creature called a Crinoid .