Santorini is essentially what remains after an enormous volcanic explosion that destroyed the earliest settlements on what was formerly a single island, and created the current geological caldera. A giant central lagoon, more or less rectangular, and measuring about 12 by 7 km (7.5 by 4.3 mi), is surrounded by 300 m (980 ft) high steep cliffs on three sides. The main island slopes downward from the cliffs to the surrounding Aegean Sea. On the fourth side, the lagoon is separated from the sea by another much smaller island called Therasia; the lagoon is connected to the sea in two places, in the northwest and southwest. The water in the centre of the lagoon is nearly 400 m (1,300 ft) deep, thus making it a safe harbour for all kinds of shipping. The island's harbours all lie in the lagoon and there are no ports on the outer perimeter of the island the capital, Fira, clings to the top of the cliff looking down on the lagoon. The volcanic rocks present from the prior eruptions feature olivine and have a notably small presence of hornblende.