A rock deep underground is primarily affected by pressure from the weight of the overlying rock and heat from the Earth's interior. Here's why:

* Pressure: The deeper you go underground, the more rock is above you. This immense weight creates immense pressure on the rocks below, pushing them together.

* Heat: The Earth's core is extremely hot, and this heat radiates outwards. The deeper the rock, the hotter it will be.

These two factors can cause significant changes in the rock:

* Metamorphism: The intense pressure and heat can cause rocks to transform into different types of rock. This is called metamorphism.

* Deformation: The pressure can cause rocks to fold, bend, or fracture. This is called deformation.

While other forces like gravity and tectonic movement also play a role, pressure and heat are the most significant forces impacting rocks deep underground.