Pre-"Trainspotting", pre-"Born Slippy", pre-Techno, Underworld were a band. They even used, god forbid, guitars!!!
In my estimation, while Underworld have been hugely successful in the acid house/rave music scene in their Mark II lineup, they have all but buried their Mark I lineup. From what I have read, they acknowledge that part of their history enough to identify it as "Mark I", but they must be embarrassed about it as the music is out of print and very hard to come by.
In the view of this writer, the Mark 1 period was worthy of more than a cursory listen. There is plenty of great music within, especially on their first album under the Underworld moniker, "Underneath the Radar". The title track from the album still gets played in Australia on classic hits radio. The single barely troubled the UK charts at all, made #74 in the US and #5 in Australia. The album also sold well and is full of great tunes, while not all as strong as the lead single.
Here for your enjoyment, is Underworld's first success in Australia, "Underneath the Radar".