That statement is mostly true. Geologists cannot directly observe the Earth's interior in the same way we can observe the surface. The Earth's interior is under immense pressure and heat, making it inaccessible to direct observation.

However, geologists have developed indirect methods to study the Earth's interior, such as:

* Seismic waves: Studying how seismic waves from earthquakes travel through the Earth allows us to infer the composition and structure of different layers.

* Volcanic eruptions: Studying the composition of lava and other volcanic materials provides information about the Earth's mantle and crust.

* Gravity measurements: Variations in gravity can indicate differences in the density of the Earth's interior.

* Magnetic field: The Earth's magnetic field is generated by the movement of molten iron in the outer core.

These methods allow geologists to create models and hypotheses about the Earth's interior, even though we cannot directly see it.

So, while geologists cannot directly observe the Earth's interior, they use various indirect techniques to understand its composition, structure, and processes.