Energis was a £10B UK telecom — it collapsed in 2002 when the dot-com bubble burst

Energis was formed in July 1991 as Telecom Electric, a demerger from the UK's National Grid Company. It built a national optical fibre network strung along the National Grid's overhead power transmission lines — a clever infrastructure shortcut that let it bypass the cost of digging up roads. The company floated on the London Stock Exchange and NASDAQ in October 1997 and became a FTSE 100 constituent at the peak of the dot-com boom, with a market capitalisation exceeding £10 billion.

During the telecom gold rush of the late 1990s, Energis borrowed heavily to acquire other companies, including Planet Online and Germany's ISION AG. The strategy assumed that demand for wholesale telecom capacity would keep growing at 100% per year. When the dot-com bubble burst in 2000–2001, telecom capacity prices collapsed and Energis was left with debt it could not service from its shrinking revenue.

By July 2002, Energis was insolvent. It was placed into administration — the UK equivalent of Chapter 11 — and its UK operations were immediately transferred to a new holding company called Chelys, owned by its banks. Shareholders were wiped out. The company that had been worth over £10 billion at its peak was sold to Cable & Wireless in 2005 for £594 million.

The collapse of Energis was part of a wave of UK telecom failures in the early 2000s that included Atlantic Telecom, Thus, and Pipex. The sector had raised tens of billions of pounds building competing fibre networks, most of which were never needed at the prices their builders had assumed. Energis was one of the largest casualties of the UK's telecom bubble.