  We consider a nonlinear wave equation with nonconstant coefficients. In
particular, the coefficient in front of the second order space derivative is
degenerate. We give the blow-up behavior and the regularity of the blow-up set.
Partial results are given at the origin, where the degeneracy occurs. Some
nontrivial obstacles, due to the nonconstant speed of propagation, have to be
surmounted.
