The recently discovered extended TeV source HESS J1834-087 is associated with both a diffuse X-ray enhancement and a molecular cloud, projected at the center of an old radio supernova remnant, G23.3-0.3 (SNR W41). New H I observations from the VLA Galactic Plane Survey (VGPS) show unambiguous structures associated with W41 in the radial velocity range of 53-63 km s-1, so we obtain for W41 a distance of 4 ± 0.2 kpc. A new higher sensitivity VGPS continuum image of W41 at 1420 MHz shows faint emission in its eastern part not detected by previous observations, so we provide a new angular size of 36' × 30' in the b-l direction (average radius of 19 pc). We estimate for W41 an age of ~105 yr. A new XMM-Newton observation reveals diffuse X-ray emission within the HESS source and suggests an association between the X-ray and γ-ray emission. The high-resolution 13CO images of W41 further reveal a giant molecular cloud (GMC) located at the center of W41, likely associated with W41 in the radial velocity range of 61-66 km s-1. Altogether, the new observations can be interpreted as providing the first evidence that an old SNR encounters a GMC to emit TeV γ-rays in the cloud material.