This is a view of the floor of the El Chichon caldera. This caldera formed during the very explosive El Chichon eruptions of 1982 and is a few hundred meters deep and about a kilometer wide. The caldera is filled with a shallow acidic lake that has a blue-green color, common to caldera lakes elsewhere. This color is partly due to the huge amount of dissolved minerals in the acidic water, and partly due to lots of fine, light-colored ashy sediment that is continuously stirred up by boiling areas. You can easily see a number of fumaroles around the base of the caldera walls.
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