The Underground Secret Base is a location in Fahrenheit: Indigo Prophecy and is an abandoned subway station underneath New York City that is used as a base by The Invisibles.
The base is visited by Lucas Kane and Carla Valenti, and Markus Kane if he survived an earlier attack, after a fight with the Oracle after finding Jade, the Indigo Child, at a Catholic orphanage. The leader of the Invisibles, Bogart, explains the situation of the Orange and Purple Clans to the group. They stay the night there, a day before the final battle, where Lucas and Carla sleep together and conceive a child and who, if The A.I wins the final battle, will become the Second Indigo Child.
Carla can find a broken radio and repair it to find information on what is going on in the world.