We consider inverse problems in space-time (M, g), a 4-dimensional Lorentzian manifold. For semilinear wave equations g u + H(x, u) = f , where g denotes the usual Laplace-Beltrami operator, we prove that the source-to-solution map L : f → u|V , where V is a neighborhood of a time-like geodesic µ, determines the topological, differentiable structure and the conformal class of the metric of the space-time in the maximal set where waves can propagate from µ and return back. Moreover, on a given space-time (M, g), the source-to-solution map determines some coefficients of the Taylor expansion of H in u .