  Most old supernova remnants (SNRs) in the Milky Way have not yet been
identified. Considering their large potential number and the sufficient
momentum-energy transfer to the interstellar medium (ISM), they are a key part
of our understanding of the overall role of SNRs in the ISM. Here we report our
discovery of an expanding molecular shell identified by CO line observations,
namely G16.11-0.51. It covers a known SNR, specifically G16.0-0.5, and is
larger in size, i.e. 0.56 deg over 0.20 deg. Based on its spatial and kinematic
structures, weak nonthermal radio-continuum emission, and derived physical
properties, we suggest that it is an old SNR. At a systemic velocity of +41.3
km s^-1, the best estimated kinematic distance of G16.11-0.51 is ~3.2 kpc,
implying its radius of about 15.6 pc. The age of G16.11-0.51 is estimated to be
greater than ~10^5 yr, and, in a dense molecular environment, it has formed
dense and thin shell layers. The kinetic energy of the expanding molecular gas
of G16.11-0.51 is about 6.4x10^49 erg, accounting for approximately six percent
of the initial SN explosion energy. Although old SNRs have essentially become
cold and hard to detect, our discovery suggests that they can be found by
searching for CO line emissions.
