  The geodesics followed by cosmic microwave background (CMB) photons show
different behaviours depending on the geometry of space. Namely, the effect of
`mixing geodesics' predicts a distinct signature in CMB maps:
threshold-independent elongated anisotropy spots in negatively curved
geometries. We have found statistically significant sign for spot elongation in
the COBE four year maps. This can be a direct indication for the negative
curvature of the Universe.
