Browsing the Internet, I had an idea.
There are currently prototypes of “3D printers for printing houses“. As an extension to this, and especially interesting for the developing world: how about a robot that can autonomously dig a house underground, consuming just water and electricity and needing nothing in addition to build the full house? It would be able to create no-cost housing anywhere.
Here is how:
The digbot / underground 3D mill will use a water jet cutter for cutting the soil, maybe photovoltaics for electric energy, and be totally autonomous, controlled by a CAD drawing. It will harvest stones from the cutting area, cut them with the water jet also, and use them to build the walls and ceiling (employing a vault for stability). It should be a self-moving tracked vehicle. It should also be able to carry the cut-off material to the outside itself, or this should be delegated to another self-moving tracked robot.
The underground facilities produced that way can be living space in developmnet countries; in many parts of Africa, no inner anti-groundwater equipment is necessary, as the groundwater level is below 30m; else, a waterproof full inner plastic sealing, insulation and pump is the simplest form. The bot also should be able to create its own abrasive for water jet cutting from hard stones it finds, and to create sand etc. from the stones for mixing concrete, and to apply the concrete for connecting stones (only if necessary) and for creating some. It is no problem if the bots operate quite slow, as it is an unattended, unmanned build process.
The first step to realize this invention would be a tunnel-cutting robot, which can be a shield-shaped one using sprayed concrete and transporting the cut-off stuff on a band conveyor, and receiving concrete, abrasive, electricity etc. likewise in tubes.
Oh, by the way: here are some prices for used water jet cutters.