I think that primarily, as others have said, we need sunlight. However, one can introduce sunlight into an underground structure with various skylights, including those that use mirrors to bring the light just about anywhere underground.<p>It’s mostly the excavation, drainage, and more robust structural requirements that increase the cost of construction of an underground house to a point where it outweighs the benefits of such a house. One would want to locate any underground structure on a site that is well above the water table, and well above a flood zone - so again, the cost of locating such a plot of land increases the difficulty and expense of building such a home.<p>However, I think that there are advantages to residences that are - half-buried. In other words, built into a hill on a slope. The advantages to such a house would be significant if it were sited correctly with a well-engineered drainage system. For example, in the Northern hemisphere the energy savings of a house on a slope with a glass curtain wall facing south would significantly reduce heating and cooling requirements regardless of latitude. Temperatures within the structure would be pretty constant year round. In winter, when the sun recedes to the south and stays lower to the horizon, sun would shine in, and warm, the house for a major portion of the day. In the summer as the sun rises more to the East and is higher in the sky, the earth surrounding the house except for its south-facing wall would insulate the house from radiant heat, and the cooler temperature of the earth would also keep the house from warming up much.<p>Our summer house in New Jersey faces South, and that long side of the house is windows the entire length of the house, and about half of the surface area of that wall. In winter, on a sunny and below freezing day, the temperature inside that long room can get as high as 80 degrees F (27 C). Midday the furnace doesn’t come on at all (unless it’s a very windy day). The basement is at ground level on the south side, completely underground on the north side. The basement maintains a pretty steady temperature year-round. Had this house been built in the past 20 years rather than 60+ year ago it would have had solar panels running fans to use the basement coolness to air condition the house on the (relatively few) hot days in summer, and distribute the excess heat in winter throughout the house. As it is, it’s an extremely energy-efficient structure.