A group of naturalists surveying the Maerdy colliery site in the Rhondda Valley has unearthed a new species of arthropod.
The brown-hued millipede has been nicknamed the 'Maerdy Monster' and is thought to be the first arthropod found in Britain for over 33 years.
The inspect was lying beneath a heap of stones and was hailed by experts as "hugely significant".
Scientist Liam Olds said: "It's not every day that you find a species new to science. It shows you don't have to go to the Amazon to find new things."
The scientific team which included millipede expert Christian Owen, were cataloguing different types of life forms at the former colliery in December 2016 when they came upon the 4cm creature.
"Under these stones and sleepers were brown millipedes unlike anything Christian had seen before," Olds said.
"He made the assumption these millipedes were merely an alternative colour form of an already existing but rare species known from numerous sites in South Wales.
"But after putting them under his microscope he recognised they were a completely different species."
The millipedes were dispatched to Dr Jörg Spelda at the Bavarian State Collection of Zoology in Germany, who established that the millipedes were "new to science".
Conserving former coal mines was necessary for science, said Olds, who is based at the National Museum of Wales. "Over the past several decades, these sites have become increasingly important places for wildlife."
Research will start to find out more about the 'Maerdy Monster' species, named after the colliery it was found in, and it will then be given a scientific name.
Steve Gregory, of the British Myriapod and Isopod Group (BMIG), said the species is "unknown and undescribed".
It belongs to the Turdulisoma and Haplobainosomatidae family and it is unclear how the creatures came to Wales, particularly since it is usually found in the Iberian Peninsula.
Gregory added: "It is not known whether this is a genuine native species, having colonised after the last Ice Age, or accidentally introduced into south Wales."
Millipedes are some of the oldest known land animals, and there are prehistoric specimens which grew to over 2m (6ft 7in) in length.