  Casimir energy in presence of a weak gravitational field is discussed taking
into account the issues related to energy and its conservation in a curved
background. It is well-known that there are inherent difficulties in defining
energy in General Relativity, essentially due to its non-localizability. Using
the concept of quasi-local mass and energy, it is shown that it is possible to
attribute a Tolman mass to a massless scalar field confined to a Casimir
cavity. Such non-local mass coincides - as expected - with the Casimir energy.
The present approach offers an alternative derivation of the vacuum buoyancy
force acting on a Casimir cavity, confirming the results presented by Calloni
{\em et al.} in a series of papers devoted to explore the possibility of
experimentally weighting the Casimir vacuum (the so-called Archimedes
Experiment).
