Deep Fission is developing an ultra-deep borehole reactor concept — placing a small reactor 1-2 km underground in a drilled borehole, dramatically simplifying siting, reducing seismic risk, and eliminating the need for a conventional containment building. Selected for the DOE Reactor Pilot Programme in August 2025. The underground concept could unlock nuclear deployment in locations where surface siting is impossible due to land constraints, population density, or regulatory requirements.