Living in the Midwest, where heating homes with propane is common, I periodically see reports in the local paper that yet another unoccupied house has exploded. They often note that the roof was found in the basement, while the walls were spread some distance into the neighboring fields.

Roof-in-basement syndrome begins when propane, which is heavier than air, leaks and fills up the lower levels of a house, mixing with the home's ambient air. Once the layer of gas reaches the height of a pilot light in a stove, it triggers a huge explosion. (The refinery adds a very strong odor to propane, so if people are home, they typically notice the leak before the concentration approaches a dangerous level.)

Why does the same gas that burns gently in a furnace turn into an explosive menace when it leaks? Without oxygen, propane can't burn. You could light a match inside a propane tank, and nothing would happen. Furnaces control the rate of burning by mixing propane with air slowly as it burns. But premix large amounts, and the results are very different.