The nature of dark matter and properties of neutrinos are among the most
pressing issues in contemporary particle physics. The dual-phase xenon
time-projection chamber is the leading technology to cover the available
parameter space for Weakly Interacting Massive Particles (WIMPs), while
featuring extensive sensitivity to many alternative dark matter candidates.
These detectors can also study neutrinos through neutrinoless double-beta decay
and through a variety of astrophysical sources. A next-generation xenon-based
detector will therefore be a true multi-purpose observatory to significantly
advance particle physics, nuclear physics, astrophysics, solar physics, and
cosmology. This review article presents the science cases for such a detector.