Maars form when rising magma meets groundwater, triggering an explosion. The dark ash from the eruption fell around this maar in a tephra ring. (more below)

This maar is one of the Ukinrek Maars, in southwestern Alaska. It's about 300 meters across and has a low, wide cinder cone around it made of dark volcanic ash.

Maars form as a result of volcanic eruptions, but what's special about them is that the eruption is actually a steam explosion. Rising magma may come up fast enough that the groundwater can't get out of its way. In that case, the water at some point flashes into steam.

That's the same mechanism that creates geysers, only much more energetic. Eruptions that involve water are classified as phreatic ("free-ATtic").

When maars erode, the lava structures inside emerge, looking quite different from the usual picture of a volcano (see the tuff ring and tuff cone pictures for examples).