The discussion centers on a massive water line explosion that occurred outside the Toronto Skydome during a baseball game. Participants speculate that the explosion may have been caused by a drainage line under high pressure due to surges from street drains and inadequate venting systems. Theories suggest that air compressibility and improper hydraulic testing could have contributed to the incident. One commenter highlights the dangers of not properly purging air from pipes during pressure tests. Overall, the exact cause remains unclear, but several plausible explanations are being debated.