  Fine structure of the density distribution in giant molecular clouds (GMC)
around W43 (G31+00+90 km/s at $\sim 5.5$ kpc) was analyzed using the FUGIN$^*$
CO-line survey at high-angular ($20''\sim 0.5$ pc) and velocity (1.3 km/s)
resolutions ($^*$Four-receiver-system Unbiased Galactic Imaging survey with the
Nobeyama 45-m telescope). The GMCs show highly turbulent structures, and the
eddies are found to exhibit spherical bubble morphology appearing in narrow
ranges of velocity channels. The bubbles are dark in radio continuum emission,
unlike usual supernova remnants (SNR) or HII regions, and in infrared dust
emission, unlike molecular bubbles around young stellar objects. The CO bubbles
are interpreted as due to fully evolved buried SNRs in molecular clouds after
rapid exhaustion of the released energy in dense molecular clouds. The CO
bubbles may be a direct evidence for exciting and maintaining the turbulence in
GMCs by SN origin. Search for CO bubbles as "dark SNRs" (dSNR) will have
implication to estimate the supernova rate more accurately, and hence the star
formation activity in the Milky Way.
