A nuclear meltdown is sometimes called the "China syndrome", which refers to a scenario, not meant to be taken literally, where a reactor core could melt through the Earth "all the way to China". The movie "The China Syndrome" is named after this scenario. There is no way such an event could happen in the real world. A reactor core could not melt through the Earth's crust, and even if it did melt to the center of the Earth, it would not go back up to the surface against gravity.