A porpoise has been sighted in the Manchester Ship Canal, more than 32km (20 miles) inland.
Porpoises are usually found at sea
The 1.5m (5ft) long Harbour Porpoise was first seen on Thursday morning in the water and is currently being monitored by marine experts.
A spokesman for the British Divers Marine Life Rescue team said it was unusual for a porpoise to travel so far upstream, but it appeared healthy.
It was spotted close to Latchford Lock in Warrington, Cheshire.
The spokesman said: "When last sighted, the cetacean was around 21 miles inland, close to Latchford Lock in Warrington.
"We are keeping an eye on the situation because there is always a danger it could get trapped in the lock gates or stranded, but so far it seems to be doing fine."
The canal is about 60m (197ft) wide and some 9.1m (29.8ft) deep and there is a good supply of fish.
The spokesman added there were currently no plans to remove the animal but that would change if it began to get into difficulties.
The porpoise, similar to a dolphin, normally lives in seawater but can survive in freshwater for several weeks without suffering ill-effects.
The Manchester Ship Canal is 56km (34.8 miles) long and runs from Eastham on Merseyside to Salford in Greater Manchester.
In 2003, the ship canal was cited as one of the fastest-improving waterways in the UK.
The Mersey Basin Campaign, a group set up 20 years ago to clean up the Mersey basin and its tributaries, said the porpoise was proof the North West's waterways had turned the corner.
Chief executive Walter Menzies said it was the latest in a string of wildlife to be spotted in the area.
"There have been otters, salmon - even an octopus - in the Mersey Estuary," said Mr Menzies.
"Hard work and ingenuity have started to pay off and a cleaner, healthier system of rivers and canals is emerging for the people and wildlife of the region."