  Given an initial distribution of sand in an Abelian sandpile, what final
state does it relax to after all possible avalanches have taken place? In d >=
3, we show that this problem is P-complete, so that explicit simulation of the
system is almost certainly necessary. We also show that the problem of
determining whether a sandpile state is recurrent is P-complete in d >= 3.
  In d=1, we give two algorithms for predicting the sandpile on a lattice of
size n, both faster than explicit simulation: a serial one that runs in time
O(n log n), and a parallel one that runs in time O(log^3 n), i.e. in the class
NC^3. The latter is based on a more general problem we call Additive Ranked
Generability. This leaves the two-dimensional case as an interesting open
problem.
