Here's why:

* Altitude is the height above sea level. 0 altitude means you are at sea level.

* Latitude measures your position north or south of the equator. 0 latitude is the equator itself, a circle that goes around the Earth at its widest point.

If you imagine a line extending from the North Pole through the Earth's center to the South Pole, the equator would be a circle around the Earth that intersects this line at the Earth's center. Therefore, the point where 0 altitude and 0 latitude meet is at the center of the Earth.