Complex sentence: ATMnet was a regional Internet service provider (ISP) located in San Diego, California, United States, formed in 1994 and operated until its purchase by Verio in November 1997 as part of their national roll-up of regional ISPs.
Split into simpler sentences: ATMnet was a regional Internet service provider (ISP) located in San Diego, California, United States. It was formed in 1994 and operated until its purchase by Verio in November 1997 as part of the latter's national roll-up of regional ISPs.