how to flatten out the middle part of landscape I made a landscape using ANT landscape add-on. Here's what I have:

What I want to do is to flatten out the middle part, to render sth like a valley. Specifically, the area that I circled below: 

How do I go about doing this? (btw I don't necessarily have to use ANT landscape). 

EDIT:
If I use S+Z+0, I get this: 
where vertices are not at z=0, instead just levels the vertices I've selected. How can I set vertices to 0?

EDIT2: I tried setting the z values to 0 for the selected vertices. As yo can see they are stretched equally but are not at z=0. In fact, the top right panel seems to set the 'median' of the vertices, not individual vertices... How do I set them to 0 using to transform tool?
Select the vertices you want to be leveled, set 3D cursor to world origin, change pivot point to 3D cursor, S+Z+0. Don't forget to reset pivot point, or other things could go wrong. Hope this helps :)