A railway tunnel has been examined in an attempt to identify what might be turning a harbour's water orange.
Discoloured water started entering Whitehaven Harbour in West Cumbria from a culvert in Queens Dock at the end of last year.
Network Rail engineers carried out tests in Bransty Tunnel between Cockickle and Whitehaven overnight.
Chris Halpin from the company said the situation was "really frustrating" for residents and businesses.
"We're really working closely with the Coal Authority and the Environment Agency (EA) to try and find the source of this water at the outset," he said.
"That will unlock what we need to do next."
The firm would share the test results as soon as they were available and then "we all need to get our heads together collectively and figure out how we're going to tackle this", he added.
Excess rainwater from the railway has been channelled into the harbour via the Queens Dock culvert for decades but, in late 2022, the colour changed and the reason remains unknown.
It is believed water from old mine workings may be coming into the tunnel.
Whitehaven Harbour Commissioners chief executive Deanne Shallcross said it was "really frustrating" it had been happening for a year.
"How much longer are we going to have to suffer?" she said.
In the short term, the culvert should be blocked off or diverted, she added.
EA area manager Pete Miles said the environmental risk was "fairly low".
"There is a lot of dilution and the harbour is open to the sea, so the tide's coming in and out," he said.
"The issue is primarily the discolouration, the visual and amenity aspect in the marina, for the marina users and the wider town of Whitehaven."
The body took samples from Queens Dock in December 2022 and February 2023 and said they showed "increased metals in the water".
Network Rail has already carried out technical and hydrogeological surveys on land above the railway tunnel and the immediate area on the Cumbrian Coast line.
It plans to carry out more tests inside the tunnel over the weekend.