Quantum mechanics is a foundation of physics, chemistry and materials science. Still, there is an ongoing debate about the emergence of the classical, macroscopic world from the well-understood microscopic world of quantum mechanics. We contribute to this discourse by demonstrating quantum superposition of massive particles at the distance (0.5 m) and time scales (2 s) of everyday life, thereby advancing the state-of-the-art of atom de Broglie wave interferometry by nearly two orders of magnitude . In addition to testing a central tenet of quantum mechanics, we pave the way for new precision tests of gravity, including the possible observation of gravitational waves and tests of the equivalence principle. In related experimental work, we demonstrate that entangled clusters of approximately 1000 atoms can be used to achieve 10-fold improvement in the sensitivity of quantum sensors based on atomic transitions; the levels of performance achieved could not have been realized with any competing (non-entangled) method.