Tiny reef-building worms have returned to the north Wales coast after a break of 60 years.
In just two years the creatures have created a 350m (1,148 ft) long reef on the beach in Llanddulas, between Old Colwyn and Abergele, Conwy.
The Countryside Council for Wales (CCW) said the return of the worm was an encouraging sign for the environment.
In Britain honeycomb reefs are more often found on the west coast between south Devon and the Clyde.
Gabe Wyn, CCW's intertidal team leader said: "Many areas of honeycomb reef have been lost from our shores, possibly due to pollution, coastal engineering work or even bait digging.
"But the reef worm's return to Llanddulas means that the conditions here must be just right for it and is an encouraging sign about the health of the environment along this part of the Welsh coast."
Honeycomb worms build the reef by creating tightly packed tubes from sand and shell on moderately exposed shores.
Although quite brittle, the reefs provide stability for a mix of other species including anemones, barnacles, limpets, winkles, whelks, mussels and crabs.
Dr Lyndsey Dodds, WWF Cymru's marine officer, said it was delighted by the news of the honeycomb worms return.
"The occurrence of honeycomb worm reefs, and the animals they support, was a major factor that led the Menai Strait to be identified as a ?hotspot? for biodiversity in a WWF report last year.
"It is exciting to learn that this rare and important habitat-forming species is returning to other areas in Wales," said Dr Dodds.
"We now have a second chance to properly protect the fragile reefs from potentially damaging activities.
"WWF Cymru has been calling for a UK Marine Act for many years to deliver a network of marine protected areas that will protect Welsh species and habitats."