  The underlying reason for the existence of gravitational entropy is traced to
the impossibility of foliating topologically non-trivial Euclidean spacetimes
with a time function to give a unitary Hamiltonian evolution. In $d$ dimensions
the entropy can be expressed in terms of the $d-2$ obstructions to foliation,
bolts and Misner strings, by a universal formula. We illustrate with a number
of examples including spaces with nut charge. In these cases, the entropy is
not just a quarter the area of the bolt, as it is for black holes.
