Kärdla sits inside a meteorite impact crater, though you would never guess it from walking the streets. The crater is about 4 kilometres in diameter and was formed roughly 455 million years ago. The original bowl has been largely filled by sediment and vegetation over the geological ages, leaving only a slight depression and the circular arrangement of the crater rim visible in the landscape.
The crater is not a tourist attraction in the conventional sense — there is no visitor centre, no signage on the street, and most of the crater floor is now occupied by the town itself. What you can do is walk to the rim, which rises gently on the north side of town, and look back across Kärdla to appreciate the topography. The Hiiumaa Museum has a display on the crater's geology and discovery. For anyone interested in impact structures, this is a genuinely significant site — one of the better-preserved Ordovician impact craters in Europe.