  We prove that the Dirichlet-to-Neumann map of the linear wave equation
determines the topological, differentiable and conformal structure of the
underlying Lorentzian manifold, under mild technical assumptions. With more
stringent geometric assumptions, the full Lorentzian structure of the manifold
can be recovered as well. The key idea of the proof is to show that the
singular support of the Schwartz kernel of the Dirichlet-to-Neumann map of a
manifold completely determines the so-called boundary light observation set of
the manifold together with its natural causal structure.
