Turning a complex surface into a volume I have a very detailed mesh (terrain surface), looking like this:





That I would like to give some volume to. A little bit like this:



However, I'm super new to Blender (especially in the modeling department), and I'm wondering if there a any modifier that would take care of this. Once again, the mesh is very detailed, so I would like to use a solution that would use few vertices, hopefully.

If no modifier exists for this kind of operation, how would one model this surface into a volume?

Thanks!
It's actually possible with just 4 verts. Select the corner verts.



Extrude them down with EZ.

Scale them to the same height if needed using SZ0RETURN



Turn them into a face using F.



Select everything with AA and hit F again.



The fill tool is smart enough to fill the holes separately and only generates 4 more ngons.

While my mesh was fine, you can play it safe and hit CTRLN to recalculate the normals. Make sure to give kheetor's comment an upvote.