Loch Shiel is 28 km long and up to 120 m deep, the fourth-longest loch in Scotland. Once a sea loch, it was cut off by glacial debris at the end of the last ice age and now sits less than 10 metres above sea level, draining to the sea through the short River Shiel near Castle Tioram. At the Glenfinnan end it is narrow and enclosed by mountains; further southwest it opens out into bog and rough pasture.
Boat trips from the jetty at Glenfinnan run down the loch in summer, and forestry tracks follow the eastern shore. The wooded islands hold fragments of ancient forest, and it's worth watching for eagles and otters.