Extreme environments can induce profound changes in materials. At the high pressure and temperature conditions found within Earth’s upper mantle, graphite transforms from a soft, opaque semimetal into diamond, a transparent, superhard electrical insulator and thermal conductor. While this transformation in carbon is dramatic, it is far from unique. As a rule, high pressure impacts the physics and chemistry of all materials and produces unexpected results. Our research focuses on using extreme environments to answer key questions about Earth and planetary interiors and to provide guidance for the design of new materials for energy-related applications. The diamond anvil cell coupled with a suite of in-situ analytical probes provides an excellent window into our planet and enables us to discover novel phenomena and new materials over a wide pressure-temperature-composition space.