Kīlauea has a large summit caldera, measuring with walls up to 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 the summit caldera likely formed over several centuries, with its construction estimated to have begun about 500 years ago, and its present form was finalized by a particularly powerful eruption in 1790. A major feature within the caldera is Halemaʻumaʻu, a large pit crater and one of Kīlauea's most historically active eruption centers. The crater is approximately in diameter and deep, but its form has varied widely through its eruptive history; the floor of Halemaʻumaʻu is now mostly covered by flows from its 1974 eruption.