A passing hiker found mystery black worms stuck on rocks in a waterfall pool.

Footage shows the bizarre dark patch of creatures visible through the crystal clear stream in Chiang Mai province, Thailand, on June 3.

When the sightseer Kanniga Sakorn went closer, they looked like creepy, slimy bugs that averaged an inch long each, clutching on the bottom.

Kanniga said: ‘I went to a waterfall to enjoy nature, but it gave me more questions than answers. I don't know what those were living in the water. I found them a bit scary.'

Although the creatures have not been identified, they closely resemble black fly larvae commonly found in rivers and streams.

Black fly larvae are filter feeders and eat the tiny organisms that float in the water. Their fan-like mouth brushes create a current that sweeps food towards them.