Very few people know about this, including me prior to today. I don't know why this hasn't been discussed more, as something like this today would be HUGE news.
In 1937, there was an oil boom in the city of New London, Texas. With oil money, a new school was constructed, and an underground natural gas line was used to heat the school.
The line leaked, and a spark from a shop class ignited the gas. The entire building exploded, with over 600 people inside.
Over 300 people died (almost all students, with a few teachers/administrators as well), with the other 300 or so being injured.
They did not have an exact number of deaths, between the poor record keeping at the time and the fact that there were many migrant oil workers in town.
The gas was untreated, so there was no odor and people had no way to tell it had leaked.
Today's natural gas is treated with an intentional odor, so you can smell a gas leak and take action. Many people think that natural gas' odor is the regular odor of the gas, but in fact it's a chemical called mercaptan which is added so you can detect gas leaks with your nose.
As an aside, a young Walter Cronkite (21 years old at the time) covered this as one of this first journalism assignments.