Yes, and there still is! For approximately 400,000 years, volcanic eruptions here built up a 10,000 - 12,000 foot mountain now called Mt. Mazama. 7,700 years ago, the volcano erupted in a cataclysmic eruption. During this eruption, so much material was evacuated from the internal magma chamber that afterwards, there was not enough left to support the remaining mountain. It collapsed and created the hole - the caldera - that we now see today half filled with water.