You're likely thinking of the lunar maria, which are large, dark, flat plains on the Moon's surface. However, these areas are not invisible from Earth.

The maria are visible as dark patches when we look at the Moon, and they are actually the reason why we see the "Man in the Moon" pattern.

Here's the catch: The lunar maria are not all visible at the same time due to the Moon's rotation. As the Moon orbits the Earth, we see different parts of its surface.

So, while some parts of the maria might be hidden from view at any given moment, they are not permanently invisible from Earth.