Radial geodesics as a microscopic origin of black hole entropy. I:   Confined under the Schwarzschild horizon
  Causal radial geodesics with a positive interval in the Schwarzschild metric include a subset of trajectories completely confined under a horizon, which compose a thermal statistical ensemble with the Hawking-Gibbons temperature. The Bekenstein--Hawking entropy is given by an action at corresponding geodesics of particles with a summed mass equal to that of black hole in the limit of large mass. 
V.V.Kiselev