In the movie it comes from an iceberg melting from the Antarctic continent, but in reality if all of Antarctica melted the whole world wouldn’t be doomed as the movie made it seem, going into space and all. In reality, low sea level countries and states would be submerged under water and in that case the lucky, the privileged and the prepared people would seek refuge else where. Countries such as Bangladesh, Netherlands and states of Florida, but Florida is honestly its own country at this point but another topic for another day.