  The energy of gravitating systems has been an issue since Einstein proposed
general relativity: considered to be ill defined, having no proper local
density. Energy-momentum is now regarded as \emph{quasi-local} (associated with
a closed 2-surface). We consider the pseudotensor and quasi-local proposals in
the Lagrangian-Noether-Hamiltonian formulations. There are two ambiguities: (i)
many expressions, (ii) each depends on some non-dynamical structure, e.g., a
reference frame. The Hamiltonian approach gives a handle on both problems. Our
remarkable discovery is that with a 4D isometric Minkowski reference a large
class of expressions---those that agree with the Einstein pseudotensor's Freud
superpotential to linear order---give a common quasi-local energy value. With a
best-matched reference on the boundary this value is the non-negative Wang-Yau
mass.
