  An analogy between the subtraction procedure in the Gibbons-Hawking Euclidean
path integral approach to Horizon's Thermodynamics and the Casimir effect is
shown. Then a conjecture about a possible Casimir nature of the Gibbons-Hawking
subtraction is made in the framework of Sakharov's induced gravity. In this
framework it appears that the degrees of freedom involved in the
Bekenstein-Hawking entropy are naturally identified with zero--point modes of
the matter fields. Some consequences of this view are sketched.
