  A soft X-ray shadow has been discovered on the periphery of the Loop I
supershell in the ROSAT All Sky Survey. The distance, size, geometry and
spectral data indicate that it is cast by an annular volume of dense neutral
matter which has formed in the collision of the Loop I superbubble and the
Local Hot Bubble. This is the first observation of the interaction region of
two colliding interstellar bubbles, and the results are consistent with earlier
theoretical work on colliding spherical shock waves.
