<p>Ultradeep drilling beyond nine to ten kilometers has progressed from an engineering challenge to a powerful approach for investigating deep Earth processes and subsurface resources. Early continental drilling showed that the deep crust is not cold or impermeable, but contains fluids, active heat flow, and ongoing chemical reactions. This review presents an integrated synthesis of scientific results from drilling that exceeds ten kilometers, with emphasis on recent Chinese programs. These wells document methane-rich fluids, high pressures, and high temperatures in sedimentary and crystalline rocks, indicating that hydrocarbon systems, deep fluids, and permeability persist far deeper than traditionally assumed. Integrating observations from cores, formation fluids, stress measurements, and geochemical analyses, we show that revised geothermal gradients, fault-controlled fluid pathways, and fluid–rock reactions such as serpentinization, carbonation, and hydrocarbon cracking sustain permeability at extreme depths. Ultradeep drilling thus provides insights into Earth system dynamics, deep energy resources, carbon storage, and mineral systems.</p>