Trollsjön is an artificial lake outside Tierp. It began as a mountain stream that broke out in connection with the construction of a new route for the E4. The lake’s history is tied to that road work, but the site was planned with a later use in mind: the rock cover was designed from the start to make a bathing lake once the rock-breaking work was over.

The description calls the lake artificial and gives the mountain stream as its origin. It links the stream’s breakout to the new E4 route. The rock cover is central to the design: it was planned from the beginning, with the stated aim of making the site a bathing lake after the breaking work ended. In that sequence, the lake’s intended bathing use came after the road-related work, while the rock cover was part of the original plan. Trollsjön’s defining detail is this connection between the stream, the route and the planned bathing lake.