Why so mangrove roots grow above the ground?

While walking near any lake or forest around the sea, you might have seen a different and unique feature of some trees, whose roots are not under but above the ground. They are known as Mangrove trees and they have aerial roots.

Now, the reason why have roots above the land is that they are the underground root type which needs and demands more oxygen. Under the ground, the soil is not able to support or provide enough oxygen to the roots and therefore this root system outgrows aerial roots which grow vertically up to the fresh air above the soil.