At approximately 1:07 PM on November 3, 1998, a 12" branch 600 psig steam line on a pipe bridge near the center of the Tennessee Eastman Division (TED) plant ruptured catastrophically. A 36" long section of the line upstream of a decommissioned venturi flow meter failed, resulting in a major steam leak. The leak led to the loss of 600 psig header pressure throughout the plant, resulting in the shutdown of all steam generation equipment, and the loss of production throughout most of the TED facility for a period of 5-7 days. Because this incident occurred in a section of steam piping over 30 years old, immediate concerns focused around the vulnerability of the entire network to similar failures. This paper discusses the resulting root cause analysis (RCA), and the surprising results obtained from the analysis of the ruptured pipeline, which uncovered a root cause completely different from that assumed by the team at the start of the investigation.