Heaving Ground

Remember in the E3 video where the ground heaved upwards? How did they do that?

Supposedly there is deformable terrain, but as I havent seen it the
only way I could think of would be to create many diff disp surfaces
and set up some sort of entity system to turn on off and the next on so
it sort of looks like it is deforming.

You certainly can't use displacements, as they are static. My
guess is that whatever they used was removed from the game before
release.

Could you put a func_train or func_door underground and trigger it to shoot up at the right time?

I really don't remember the exact scene from E3, so I'm not positive that the effect you wanted.

I guess it was removed from the game like so much that i miss. I wish i had never deleted brother's copy of the leaked version...

What about the env_terrainmorph entity? I haven't used it, but it certainly sounds like it should do the job.

I did a quick test with the terrainmorph entity, and it seems like it
works. The enitity is actually documented, so you're in
luck. It seems kind of glitchy though. I noticed that
displacement maps that have been morphed seem to disappear when you
stand on them. Some trial and error will be required, but there's
likely a reason it wasn't used in HL2.