The field has also seen many Surtseyan or phreatomagmatic eruptions, especially at nearshore vents, when ascending magma encountered water, such as at Mānana Island, Koko and Punchbowl; these eruptions were explosive and they are the best exposed site of Surtseyan activity in the United States. Some formed small islands when they emerged, and one vent may now be buried beneath coral deposits. At Koko Crater and Diamond Head, the amount of water interacting with the developing volcano varied over the course of the eruption, as more water entered the vent when the eruption enlarged it.