Scientists haven't actually "arrived at" the distinct layers of the Earth in the way we might think of reaching a destination. We can't travel through the Earth like we can travel through a tunnel.

Here's how scientists have learned about the Earth's layers:

* Seismic Waves: Earthquakes generate seismic waves that travel through the Earth. Scientists use seismographs to record these waves. Different types of waves travel at different speeds and can be reflected or refracted at the boundaries between the Earth's layers. By studying the patterns of these waves, scientists can deduce the composition and depth of each layer.

* Rock Samples: Scientists study rocks from the Earth's crust, which provides information about the Earth's composition and processes. They can also analyze meteorites, which are thought to be similar in composition to the Earth's core.

* Gravity and Magnetic Field: The Earth's gravity and magnetic field are influenced by the density and composition of its interior. By studying these phenomena, scientists can gain insights into the structure of the Earth's layers.

* Laboratory Experiments: Scientists conduct experiments in laboratories to simulate the conditions found within the Earth's layers. This helps them understand how materials behave under high pressure and temperature, which in turn helps them interpret data gathered from other sources.

So, while we haven't physically traveled to the Earth's core or mantle, scientists have been able to develop a detailed understanding of the Earth's internal structure through these indirect methods.