1929 Ottawa sewer explosion
On May 29, 1929, a series of explosions in the sewers of Ottawa, Ontario, Canada, killed one person. The first blast occurred just after noon in the Golden Triangle area, west of the canal; over the next 25 minutes, a series of explosions travelled the length of the main line of the sewer system.
Source: Wikipedia — 1929 Ottawa sewer explosion (CC BY-SA 4.0)