It is possible for part of California to sink into the ocean, but not all of it. About 10 million years ago (in the Tertiary Period), much of California sank beneath the waves. Then about 5 million years ago (in the Quaternary Period), parts of southern California rose out of the sea and started becoming dry land again.
Today, some areas are rising up while others are sinking down because plate tectonics are making new mountains in various places across the state while other areas are being worn away by erosion, especially near the coast where weather patterns often include high winds and heavy rains coming off of an ocean that is constantly churning with storms both large and small. The steady rise in global sea levels due to climate change also poses a threat. In the long term, as the great tectonic plates of Earth's crust continue to move and change as they have for billions of years, it is possible that the entire state of California could once again be submerged beneath water.
In order for all of California to sink into the ocean, something would need to happen along a fault line deep under the ocean floor where part of California's land mass rests on top of a subducting plate. If this were to happen, either an earthquake or volcanic eruption would eventually cause part – but probably not all – of California to rise up out of the sea as a new island continent.
