Because of the persistent presence of the ice sheet, the island is sometimes mistaken to be a part of Antarctica. Its area is 2,460 km² (950 sq mi); only a small portion of the island is free of ice and snow. The planet's southernmost active volcano, Erebus (3794 m), as well as the dormant volcano Terror (3230 m), are situated on the island. They were named by Ross after his ships HMS "Erebus" and HMS "Terror". The third highest peak is Mount Bird, and on its slopes are Shell Glacier and Endeavour Piedmont Glacier. Abbott Peak stands between Mount Erebus and Mount Bird.