Final answer:
Oceans disappear due to the subduction process where an oceanic plate is forced beneath another into the mantle, balancing crust creation and destruction.
Step-by-step explanation:
The process responsible for oceans disappearing is known as subduction. This tectonic process occurs when two plates converge, and one plate is forced beneath the other into the upper mantle, creating what is called a subduction zone. A classic representation of a subduction zone is an ocean trench such as the deep Japan trench along the coast of Asia. Through subduction, the oceanic plate that gets pushed beneath can melt and be recycled into Earth's mantle. The recycling of the old crust through subduction zones balances out the creation of new crust during seafloor spreading at rift zones, where continents are pulled apart.