They shoot molten lava and toxic gases miles into the atmosphere and when the lava is too thick to run over the top, it can create what geologists like Dr. Pat Abbott call a plug dome volcano.

Lassen Peak is the largest plug dome volcano in the world and a signature feature of the park. Need a break? Play the USA TODAY Daily Crossword Puzzle.

An extinct volcano is lying hidden in Carlsbad's Calavera Nature Preserve in north San Diego County. Welcome to Mount Calavera!