Underhill was the first Martian city, built by the First Hundred upon their arrival.

The site was chosen prior to their arrival; equipment was sent on the site with three unmanned expeditions prior to the arrival of the Ares. The settlement got its name because of its underground location beneath a hill.

In Underhill, the first farms were built, along with the first nuclear factory ("Chernobyl"), a clinic, chemistry laboratories ("alchemists' quarters") and the first industrial infrastructure. The first habitats were underground, making use of raw materials such as farm-grown bamboo. Engineer Nadia Cherneshevsky was the most active colonist in its construction.

Underhill was the de facto first capital city of Mars. As more colonies arrived and new settlements were built, the power center shifted to Burroughs.

Nearly two hundred years after its founding, the surviving First Hundred gathered again in Underhill to undergo the memory treatment created by Sax Russell, to facilitate the memory recovery process.