Stora Karlsö is a rock in the Baltic, with an area of 2.4 km², situated 6 km off the southwest coast of Gotland. Stora Karlsö is composed of fossilised coral reefs of various sizes surrounded by limestone. The reefs were formed in warmer waters 400 million years ago when Gotland was located near the Equator. Loose fossils, dead sea plants and animals turned to stone, are abundant and visitors are allowed to pick them up and study them but they must not be taken from the island.
There are more than 20 caves on Stora Karlsö, many of them situated around the small harbour of Norderhamn. The deepest cave is 25 metres and archaeological finds show evidence of human activity there as early as the Stone Age.
Except in the sloping south, the sides of Stora Karlsö are steep, 25-45 meters in some places. The highest point is 51.6 metres above sea level. In the ravine in the northern part there is a small coastal plain and also the harbour Norderhamn. There, the motor-launch, which in the summer months operates daily from Klintehamn on the main island, can put ashore visitors, staff, provisions and so on.