  M16, the Eagle Nebula, is an outstanding HII region where extensive high-mass
star formation is taking place in the Sagittarius Arm, and hosts the remarkable
"pillars" observed with HST. We made new CO observations of the region in the
12CO J=1--0 and J=2--1 transitions with NANTEN2. These observations revealed
for the first time that a giant molecular cloud of $\sim 1.3 \times 10^5$ \Msun
\ is associated with M16, which is elongated vertically to the Galactic plane
over 35 pc at a distance of 1.8 kpc. We found a cavity of the molecular gas of
$\sim 10$ pc diameter toward the heart of M16 at \lbeq (16.95\degree,
0.85\degree), where more than 10 O-type stars and $\sim 400$ stars are
associated, in addition to a close-by molecular cavity toward a Spitzer bubble
N19 at \lbeq (17.06\degree, 1.0\degree). We found three velocity components
which show spatially complementary distribution in the entire M16 giant
molecular cloud (GMC) including NGC6611 and N19, suggesting collisional
interaction between them. Based on the above results we frame a hypothesis that
collision between the red-shifted and blue-shifted components at a relative of
$\sim 10$ \kms \ triggered formation of the O-type stars in the M16 GMC in the
last 1-2 Myr. The collision is two fold in the sense that one of the
collisional interactions is major toward the M16 cluster and the other toward
N19 with a RCW120 type, the former triggered most of the O star formation with
almost full ionization of the parent gas, and the latter an O star formation in
N19.
