When you look at the seas and oceans, you might think they flow into each other, forming one big ocean. However, the boundary between a sea and another sea can be very vivid. 

The border between two oceans is like a line between two worlds.

This happens at the junction of the Pacific and Atlantic oceans, as if the two oceans meet at an invisible wall which does not let them flow into and mix with each other.

Why does it happen?

The Atlantic Ocean and the Pacific Ocean have different densities, chemical compositions, and levels of salinity and other qualities.

When the salinity of the water in the two oceans is different to a certain extent, a halocline appears. Explorer Jacques Cousteau discovered this when he was deep diving in the Strait of Gibraltar. The layers of water with different salinity looked like they were divided with a transparent film, and each layer had its own flora and fauna.

The two oceans with completely different surface tensile strengths also make it impossible for them to mix.

In addition, differences in temperature, such as between the warm waters of the Gulf Stream and the colder North Atlantic, can contribute to the phenomenon.

In China, you can also enjoy this wonderful natural phenomenon at the border of the Yellow Sea and Bohai Sea.

About The Blue World series:

The ocean covers more than 70 percent of the Earth. In this blue world, there are all kinds of marine life with surprising abilities we don't know about. In this series, CGTN guides you through the journey of exploring the amazing animals in the ocean. 

For more:

The Blue World: The mystery of the sex of green turtles

The Blue World: Brainless jellyfish

The Blue World: Half asleep, half awake dolphins

The Blue World: Never make a pufferfish angry

The Blue World: From round fish to flatfish

The Blue World: When a whale sinks into the sea

The Blue World: Sea otter's treasure chest

The Blue World: Mother octopuses' mission

The Blue World: When the beauty of coral fades

The Blue World: Whale beachings

The Blue World: A shark's sixth sense

The Blue World: The glowing sea

The Blue World: Far away from the shore

(All images via VCG)

(If you want to contribute and have specific expertise, please contact us at nature@cgtn.com.)