This is what the aftermath of the biggest eruption in recorded times looked like in 2009 (click to enlarge, or visit the linked NASA page for more information):
Before April 10, 1815, there used to be a 13,000-foot-high Fuji-like mountain here. On the 10th, Tambora blew some 36 cubic miles of rock and ash into the air, killing tens of thousands of people directly and indirectly, making 1816 “the year without a summer,” and creating other problems that ultimately led to the invention of the bicycle.
During the April 1815 eruption, when the magma chamber had emptied, the top of the mountain collapsed in on itself, forming this caldera. It’s almost 4 miles wide and over 3600 feet deep now.
Here is where Tambora’s eruption fits in among notable recent eruptions (it’s the light orange area on the big ball):
It’s still active, but on a much smaller scale (fortunately). Mount Tambora last erupted in 1967. It was put on low-level alert for several days in April 2013, but no eruption ensued.