How do i create hills in hammer?

How do i create scenery like mountains and hills in hammer?

Displacements.

Some nice tutorials about them: http://halfwit-2.com/?page=tutorials&category=19

Carve them.
Kidding, use Displacements. Remember to keep them smooth-looking!

You can also use terrain generator by Nem if you’re making a big map, I’m pretty sure Bigwig used it for gm_atomic, or he’s very patient.

Use displacement with power of grid setted to 3 ! Cause 4 casts many problems

Only on Orangebox it gets fucked up.

Turn the brushes or individual faces you want to make terrain from into displacements. Do this by using the Face Edit Tool and clicking on the Displacements Tab and then Create. This allows free manipulation of the surface of whatever brush or face you use.
Displacements can have special textures with ‘Blend’ in their name. These textures can literally blend seamlessly between two different textures, such as dirt and concrete. In other words, you can have two textures on one face.