I would suggest, that we can build into the ground, like a mine shaft vertically or horizontally - working the same way that elevators work currently, once you dig a hole you place the entrance piece that is like a mine shaft elevator.
Also you have chance to find resources inside such as metals and gems - or hidden caves with deadly creatures!
Building into the earth would be limited by a number of possible factors - depth, you would hit stone that is too hard to dig into. Water - you cannot dig into water. Map borders - borders that surround the 3D world (again by use of impenetrable rock underground). Oxygen levels - air quality will decrease further from the surface, and so building too deep will cause suffocation. Walls will require reinforcement, or they will collapse after a few minutes, returning to their original state (collapsed entrances will destroy the insides after time, effectively increasing the decay rate).
Restrictions on the building underground can also be added, by only allowing on certain terrain types, angles of digging, no building underneath existing structures (including other players/NPC structures).
There could be an underground, yes it would take some work, but it’s no where near impossible.
You could just add a world under the existing textures, that connects to the ground in certain places - this is where you can dig. If they can add other things above ground, it is a simple implementation i’m sure.
It is FAR from simple. Adding features above ground is (technically speaking) nothing like adding an underground. It’s not negativity, it’s a basic understanding on the amount of work involved. Suffice to say, it’s a gigantic task - I cannot overstate how massive, really. It may seem simple to you, but you can trust me (or not, up to you) when I tell you it isn’t.
Like I said, it’s a cool idea, but the game would’ve had to be designed with this in mind from the ground up for it to be practically possible.
Now of course, just about anything is technically possible if you throw enough money at it, but the value (while significant) is nothing compared to the cost.