  We present the detection of a bubble-like cavity traveling through a
quiescent prominence. The H-alpha emission in the cavity is >16 times smaller
than in its surroundings. The cavity propagates almost with the phase-velocity
of MHD compressive waves. We suggest a disruption of the lateral magnetic
stability. The Ca II 8542 spectra indicate a material outflow along the lines
of force up to 12 km/s.
