Dig, said the mole.

The only picture I found of the construction.

Iaman is contemplating building a house in Texas and the first question that comes up is what kind of foundation to use, which entails knowing what kind of ground he is standing on. Dirt can be excavated with a shovel. It there are some rocks, they can be pulled out. But this is central Texas, which is basically one big rock.

Digging a hole in sold rock can be a bit of a challenge for most people, but not the Texas legislature. They wanted to expand their office space, but they did not want to block the view of the capitol building, so they elected to build it underground, carving a giant hole out of solid rock.

Texas State Capitol building section through light courts, north view.

Texas State Capitol and Extension building section looking west.

Skylights for the underground Capitol Extension

In 1993, the $75 million, four-story, underground Capitol Extension was completed to the north, doubling the square footage available to Capitol occupants and providing much improved functionality. Though the extension encompasses 667,000 square feet (nearly twice the floor space of the original building), there is little evidence of such a large structure at ground level, except for extensive skylights camouflaged as planter rows, and the four-story open-air inverted rotunda. - Wikipedia