Smoo Cave (Durness)

Located about a mile east of the city of Durness, it is possible to explore the cave by boat or the path from the cliffs park. The cave features one of Britain's largest entrances to any sea cave at a height of 50 ft and is inside floodlit. It was formed by a burn running down into the back chamber, as well as sea-induced erosion. 

Smoo Cave is open throughout the year (although it only takes place between April and the end of September and only when flood conditions do not exist). The parking lot is adjacent to the highway and the cave itself is a short walk (up and down, somewhat rough-textured) from there. The visitor center is beautifully done and easily accessible toilets for visitors. There's no charge at all.