These mysterious serpents, dubbed greywyrms, are a rare but memorable sight on tube platforms.
The ashen snakes are unique to the London underground. They favour dank conditions and are usually found basking in puddles and mildewed corners. They live on a diet of tube mice, pooled water and salt and vinegar Hula Hoops.
Scientists believe that the snakes are descendants of the common adder. After centuries underground, the creatures have evolved to hunt by smell and sound alone. Their vestigial eyes are now completely blind.
Every tube line has its own distinct species of greywyrm. Serpents from the Circle line (shown above) are the least specialised and still resemble their adder ancestors. Researchers believe that exposure to open-air sections of the line has placed less selective pressure on the snakes, and so they have not evolved so far from ancestral stock.
The Northern line, by contrast, is populated by a highly divergent species. The bearded albino greywyrm (pictured above) has developed absorbent fronds to better extract nutrients from platform surface water. This variety is also much paler than its Circle line cousin. It has lost all pigmentation after centuries spent in dark tunnels. Note also the red breathing tubes — biological snorkels — which allow the greywyrm to spend prolonged periods submerged. Environmentalists fear that this species may become endangered, as Transport for London finds more efficient ways to pump ground water out of its tunnels.
It's believed that up to 15,000 greywyrms live across the Underground network. Even so, they are bashful creatures, rarely seen by the public and poorly understood by science. We'd like to make an appeal. If you spot one of these enigmatic animals on the tube, please take a photograph and share it in the comments below, or else email firstname.lastname@example.org. We would like to further document the natural history of these unique serpents before their numbers plummet further.