  The Einstein Telescope (ET), a proposed European ground-based
gravitational-wave detector of third-generation, is an evolution of
second-generation detectors such as Advanced LIGO, Advanced Virgo, and KAGRA
which could be operating in the mid 2030s. ET will explore the universe with
gravitational waves up to cosmological distances. We discuss its main
scientific objectives and its potential for discoveries in astrophysics,
cosmology and fundamental physics.
