  We study the problem of recovery the source $a(t,x)F(x)$ in the wave equation
in anisotropic medium with $a$ known so that $a(0,x)\not=0$ with a single
measurement. We use Carleman estimates combined with geometric arguments and
give sharp conditions for uniqueness. We also study the non-linear problem of
recovery the sound speed in the equation $u_{tt} -c^2(x)\Delta u =0$ with one
measurement. We give sharp conditions for stability, as well. An application to
thermoacoustic tomography is also presented.
