Otters have returned to the River Mersey, which was once one of the most polluted areas of water in Europe.
The animals' pawprints have been spotted by Forestry Commission rangers by the river near Fiddlers Ferry.
Rangers have yet to spot an otter on the riverbank and have urged anyone who photographs one to contact them.
The animals will only live in clean water so their reappearance is being interpreted as evidence that the River Mersey is now less polluted.
The river has been cleaned up over the past 25 years by The Mersey Basin Campaign, which was set up after the river was condemned in the 1980s by a government minister.
The then secretary of state for the environment, Michael Heseltine, visited the area in the wake of the Toxteth riots and said the river was "an affront to the standards a civilised society should demand of its environment".
He said: "Untreated sewage, pollutants, noxious discharges all contribute to water conditions and environmental standards that are perhaps the single most deplorable feature of this critical part of England."
Since the clean-up, animals have been gradually returning to the water.
Salmon recently returned and four species of owl have also been spotted around the Fiddlers Ferry area.
The Environment Agency has said the Mersey is now the cleanest it has been since the early 1900s.
Duncan MacNaughton, a forest ranger who helps maintain and monitor Forestry Commission land around the area, said: "To see evidence of otters returning to the estuary is incredibly exciting for everyone interested in nature conservation because it will have been more than 30 years since we have seen them there.
"So far we've seen pawprints, but not an actual otter. If anyone could capture a photograph of an otter on the site that would confirm once and for all that otters have made a comeback."
He said otters were shy animals but that there was always a chance one might be spotted.
He added: "There is so much wildlife on people's doorsteps that we want to encourage them to get out and see it for themselves.
"They can also help us record sightings in the area."