  This paper summarizes the contribution presented at the IX Marcel Grossmann
Meeting (Rome, July 2000). A simple model of spacetime foam, made by $N$
Schwarzschild wormholes in a semiclassical approximation, is here proposed. The
Casimir-like energy of the quantum fluctuation of such a model and its
probability of being realized are computed. Implications on the
Bekenstein-Hawking entropy and the cosmological constant are considered. A
proposal for an alternative foamy model formed by $N$ Schwarzschild-Anti-de
Sitter wormholes is here considered.
