Kīlauea has a large summit caldera, measuring 4 by 3.2 km (2 by 2 mi) with walls up to 120 m (400 ft) high, breached by lava flows on the southwestern side. It is unknown if the caldera was always there or if it is a relatively recent feature, and it is possible that it has come and gone throughout Kīlauea's eruptive history; what is known is that it likely formed over several centuries, with its construction estimated to have begun about 500 years ago,[16] and that its present form was finalized by a particularly powerful eruption in 1790. A major feature within the caldera is Halemaʻumaʻu Crater, a large pit crater and one of Kīlauea's most historically active eruption centers. The crater is approximately 920 m (3,018 ft) in diameter and 85 m (279 ft) deep, but its form has varied widely through its eruptive history; the floor of the Halemaʻumaʻu Crater is now mostly covered by flows from its most recent eruption, in 1974.
--- http://en.wikipedia.org/wiki/K%C4%ABlauea