Approximately 7,700 years ago Mt. Mazama let loose a lot of steam in one of the biggest eruptions in the past 10,000 years. The resulting caldera became home to the deepest lake in the United States, and the seventhdeepest in the world.

Sources:

“Crater Lake.” Oregon State University, http://volcano.oregonstate.edu/crater.... Accessed 2 August 2017.

Deur, D. (2002). “A most sacred place: The significance of Crater Lake among the Indians of Southern Oregon.” Oregon Historical
Quarterly, 1849, http://pdxscholar.library.pdx.edu/cgi.... Accessed 1 August 2017.

“Mount Mazama and Crater Lake: Growth and Destruction of a Cascade Volcano.” USGS,
https://pubs.usgs.gov/fs/2002/fs09202/. Accessed 1 August 2017.