  The extraordinary starbursts found in ultraluminous IR galaxies occur in
molecular gas concentrated in compact very massive "clouds" which we call
"Extreme Starbursts". They have one thousand times the mass but are only a few
times larger than GMCs. High-mass star formation in sufficiently dense and
massive structures does not disrupt further star formation; it is a runaway
process. Star formation remains embedded in the molecular gas and there is
little or virtually no optical-UV radiation. In the early universe extreme
starbursts may be more frequent and they may be the mode of star formation in
high redshift submillimeter sources.
