  We report on the discovery of a molecular cavity in the Norma near arm in the
general direction of Westerlund 1 (Wd1), but not associated with it. The cavity
has a mean radial velocity of -91.5 kms^{-1}, which differs by as much as ~40
kms^{-1} from the mean radial velocity of the Wd1 stars. The cavity is
surrounded by a fragmented molecular shell of an outer diameter of about 100 pc
and 10^{6} M_odot, which is expanding at velocities of 6 to 8 kms^{-1}. The
amount of kinetic energy involved in the expanding shell is ~10^{51} erg.
Inside this cavity the atomic HI gas surface density is also the lowest.
Structure of the extended Very High Energetic (VHE) gamma-ray emission,
recently reported by the H.E.S.S. collaboration Ohm et al. 2009, coincides with
the cavity. The observed morphology suggests that the inner wall of the
molecular shell is the zone of the gamma-ray emission, and not the dense gas
surrounding massive stars of Wd1 as had been speculated by the H.E.S.S.
collaboration. A likely candidate responsible for creating the observed cavity
and the gamma-ray emission is the pulsar PSR J1648-4611.
