Ubehebe Crater was formed when magma from deep in the earth rose through a fault, heating up groundwater and causing a massive steam-powered explosion...the dark layer over the exposed bed-rock is one of cinders from that event...a recent study suggests that this took place as little as 800 years ago, and that the potential is there for a similar occurrence in the future

this shot taken by Sam gives a better idea of the huge size of the crater: http://www.pbase.com/pegsam/image/162399843