The region of the Earth's interior where heat increases to the point that rocks begin to melt is the asthenosphere.

Here's why:

* Asthenosphere: This layer lies beneath the lithosphere (the Earth's rigid outer layer) and is characterized by partially molten rock. The intense heat and pressure within the asthenosphere cause some of the rock to melt, creating a semi-solid, viscous state.

* Temperature and Pressure: As you go deeper into the Earth, both temperature and pressure increase. While pressure generally inhibits melting, the high temperatures eventually overcome the pressure, leading to partial melting within the asthenosphere.

Let me know if you'd like to learn more about the Earth's interior!