  In a large sample of ULIRGs imaged with HST, we have identified a significant
subsample that shows evidence for multiple mergers. The evidence is seen among
two classes of ULIRGs: (1) those with multiple remnant nuclei in their core,
sometimes accompanied by a complex system of tidal tails; and (2) those that
are in fact dense groupings of interacting (soon-to-merge) galaxies. We
conservatively estimate that, in the redshift range 0.05<z<0.20, at least 20
(out of 99) ULIRGs satisfy one or both of these criteria. We present several
cases and discuss the possibility that the progenitors of ULIRGs may be the
more classical weakly interacting compact groups of galaxies (Hickson 1997). An
evolutionary progression is consistent with the results: from compact groups to
pairs to ULIRGs to ellipticals. The last step follows the blowout of gas and
dust from the ULIRG.
