For those that may not know, the way propane and natural gas react when leaking are quite different and present different problems. Natural gas normally has no odor, so they put that distinctive one in it so you can smell it if it does leak. Then, natural gas is (at room temperature) normally a gas that is lighter than air, so it rises. If you have a leak, it will rise up to the highest point and then, if it can't escape, fill from the top down.

But, propane is normally stored at high pressure and is generally a liquid until it is released and the pressure drops. And, it is heavier than air. When it leaks, it will fill a room from the bottom up. So, say your water heater was in the basement - the entire basement could be full of propane gas, and none of it would have yet reached the upper levels, so you may not know it. Give it a spark at the right concentrations and the basement blows up, taking the house with it.

Not to say that natural gas isn't dangerous too, but because it rises, you are more likely to smell it and take corrective action. But, if the house IS tight, and it filled from the top down, since most spark sources (like a pilot light, or furnace or other device with a pilot or an ignitor) are likely in the basement or at least on the lower floor, you may have more of the house full of gas before it ignites. Instead of blowing the basement up and taking the house with it, it blows the roof off and the walls out. I guess, neither is likely to allow the house to still stand.

FYI, similar to a smoke detector, there are gas detectors available that can warn you at the first signs of a gas leak. Something to consider when you have gas appliances in the home. First, you're not always home, and then, depending on the type, you may not notice until it has reached a dangerous level.