Thirlmere is a reservoir situated in the Lake District and built at the end of the 19th century to supply water to Manchester. There were previously two small natural lakes and two villages (called Amboth and Wythburn) in the valley, and the original plan to create the reservoir met a great deal of opposition.
The plan went ahead of course, and the dam was built between 1890 and 1894.
There are forested areas around substantial parts of the lake, mostly coniferous trees planted after the reservoir was created but also some more recent deciduous plantations, giving plenty of opportunities for woodland walks on the marked trails.
The high mountain of Helvellyn stands to the east so if you walk up one of the fells to the west of the lake eg Raven Crag and Armboth Fell you can enjoy very fine views back across Helvellyn and Thirlmere.
There is also good access to Thirlmere Lake itself, and various viewpoints and picnic spots, along the road that follows the western shore.