Yup - Lake Baikal. Holds about 20% of the world’s fresh water.

20% of world's surface fresh water.

> Surface water is water located on top of land forming terrestrial (inland) waterbodies, and may also be referred to as blue water, opposed to the seawater and waterbodies like the ocean.

You made it sound like "surface water" is literally just counting water close to the surface of the body of water, but it's basically any lake/river, so still impressive.

> Lake Baikal is the world's largest freshwater lake by volume, containing 22–23% of the world's fresh surface water, more than all of the North American Great Lakes combined. It is also the world's deepest lake, with a maximum depth of 1,642 metres (5,387 feet; 898 fathoms), and the world's oldest lake, at 25–30 million years.

They are distinguishing between water located on the surface and the water that is located in the hollow at the center of the Earth, where King Kong and dinosaurs live on to this day.

I wonder how they experience gravity down there.

If you're standing on the inside surface of a sphere with a hollow, you should not experience any gravity. It's pretty important for supporting King Kong's body-weight, normal 1g would be hell on his knee joints.

I don’t think they made it sound like that, they just used the word, if you don’t know the definition it’s on you?

In comparison to ground water, which is located in aquifers, dirt, caves, etc.