For almost 380 years, fish have not been able to swim freely in the waters, but now a special migration path for fish has been built in the new Slussen lock system.

The tunnel was built especially for fish who are weaker swimmers such as perch and pike. The path aims to diversify the fish population in Malaren, Sweden’s third biggest lake.

When building the new lock, creating a fish migration path was an environmental precondition. Builders used stones from a defensive wall from the 1540s found in archaeological digs, to create a tunnel underground.

A camera has been installed in one part of the tunnel to monitor the fish swimming through – and sometimes it catches more unexpected creatures, like an otter.

To prevent salt water from entering from the Baltic Sea and to conserve water, Malaren is regulated by opening and closing various dams and sluice gates in Stockholm.