Darkness in the Deep Sea

In the future world, the post-war era has made it impossible to survive on the earth's land. Human beings have begun to move everywhere in order to save themselves. Some go to the moon to settle, and some immigrate to Mars. However, more people cannot give up and can only start to seek refuge in places other than land. The ocean is an example, but the world of the ocean is not that simple. Originally, the deepest place was 10,000 meters, but now we cannot see the bottom even at tens of thousands of meters. Is our ocean still the familiar ocean?