NGC3603, one of the Galactic super star clusters, is an extremely active, remarkable object with high-mass star formation. Recent molecular observations with NANTEN2
have shown evidence for cluster formation triggered by a collision between two GMCs.
Numerical simulations of such a process do demonstrate formation of high-mass stars at a very high efficiency in the colliding molecular layer. I will discuss how such collisions are important in the context of high-mass star formation in galaxies and galactic evolution.