Fastest way to make big tunnels

[Tek2]

What is the fastest way to make around 15x15 tunnels into hills/mountains?

[BuffetOfLies]

In Voxed you could use Shift-E for delete ellipsoid. To select the size you could use the space bar to select two opposite points of the ellipsoid and then drill holes. Or use the numpad to stretch the hole. Shift-L does the same thing with a cylindrical log-like shape.

[MediumShadow]

if you are building in game make a 3x3 tunnel then on the very middle block dig out the middle block 2x so there is a whole 2 blocks deep then toss a nade in there make sure it stays in there

[jojoestinky]

you just subtract box volumes...

Press space bar where you want the top left of the box, then spacebar again for the Bottom right corner of the box ,then press the Delete key to subtract...

PROTIP:

After you subtract, immediately add another grey volume in its place, then subtract out the middle to make your tunnel. This way, you basically make grey tunnel walls, so it looks right.

Adding posts and columns really helps the looks and gameplay as well, it gives people places to hide, and breaks up the long tunnel, with something to look at.