Entropy of Spacelike Two-Surfaces of Spacetime
  We argue that every finite piecewise smooth spacelike two-surface of spacetime possesses an entropy which is, in natural units, one quarter of its area. However, the thermodynamical properties of a two-surface become apparent only to the observers having that two-surface as a horizon. Consequences of this result are discussed. 
J. Makela and A. Peltola