At the western tip of the Seltjarnarnes peninsula, where the Reykjavik suburbs run out into the sea, stands a small white lighthouse on a tidal islet. At low tide a sandy causeway lets you walk out to it; at high tide the island is cut off.
Because it is far enough from the streetlights to give a properly dark sky, Grótta is the most popular Northern Lights spot in the city — Icelanders themselves drive out here on auroral nights. By day the area is a bird sanctuary; in summer the eider duck nest in the marram grass and the geothermal "foot pool" by the car park is a free, lovely place to dip your feet in warm water while looking out across the bay.