How about a Unit that can creat a tunnel through hole planet. This take a while to build, but when it finished you can send your units through the tunnel and when you put it at the right location you might show up in the base of your enemy. Its like a fast travel system.
Digging through liquid is hard. That liquid being molten rock and metals makes it harder. The solid iron core would also pose a problem... and that if you could even get down that far with a stable 'tunnel', considering the pressure it's under.
And while a great video, note the things they to disregard to make it "work"; No air resistance No Friction Pole-to-pole to avoid the Coriolis Effect. Never mind the feasibility of making a tunnel through the various materials and dealing with the extreme pressures found within. Mike
Ignore the haters, it was an interesting suggestion. We'll see what gets incorporated in the final builds, but I imagine there will be many forms of unit transport available to the player.
Underground stuff can be cool, but the problem is more interface related than anything else. You have a large, opaque planet covered with interesting stuff. How is any underground thing going to be visible? More importantly, how will you know its depth, direction, where it's going, and how are you going to see it work if there's a big planet in the way? These are not as easy to answer as you might think.
I think unit teleporters which would be like masive stargates, very expensive and such, when you need to move across large distances. enormous power drain.