Abstract: Among the various class of intervening quasar absorption line systems, the origin(s) of weak Mg II absorbers have remained a mystery. It is not well understood what kind of astrophysical structures produce these absorption systems. I will present results from UV and optical spectroscopic surveys of ~100 quasar sight lines, which has yielded a full census of this absorber population over a significant fraction of the history of the universe. Based on the results we extend the proposition that a substantial fraction of these absorbers could be tracing gas at the galaxy-IGM interface, analogous to Galactic high-velocity cloud structures.<br>