The Keen of Hamar is a 50-hectare hilltop of bare, rust-coloured serpentine rock that looks almost lunar. The mineral composition makes the soil toxic to most plants, but a handful of rare specialist species thrive here — including the Edmondston's chickweed, a small white-flowered plant found in only a few sites on Unst and nowhere else in the world. Mountain everlasting, Norwegian sandwort, and stone bramble are among the other notable finds.
A short loop walk from the car park at the eastern edge of the reserve takes about an hour. The flora is best seen in late May and June. The hill itself is windy and exposed even on still days lower down; bring a jacket. Free admission, always open.