The current practice in the US is that gas appliances generally have an "igniter," and it must light a "starter" or pilot flame, the starter flame must heat a thermocouple sensor within a limited time before the main gas turns on, and on the better ones the starter flame is cut off if it doesn't light. IF EVERYTHING is installed right, heating furnaces with these features are very safe.

Hot water heaters still sometimes use a pilot light, also with a heat sensor that has to show that the pilot flame is present before the main burner can turn on, and both will turn off if the thermocouple doesn't sense a flame. With most of these it's necessary to hold an override valve open to light the pilot and until flame heat is sensed. If the override is released prematurely all the gas is shut off, but once lit the pilot burns continuously as long as there's "flame heat" at the sensor.

There are still a number of gas cook stoves that use a constantly burning pilot flame for ignition of the top burners in use, although more recent ones will shut off all the gas, including the pilot gas, if the flame goes out. Whether this is reliable may depend on how much grease has been spilled into the stove top(?). Some older stoves still required manual lighting of an oven pilot for either oven or broiler, at each use, and everything shuts off when the oven is turned off. Newer stoves most likely have electronic/piezo igniters, but we haven't been in a house piped for cookstove gas recently enough to have much info on recent practice.

A leak large enough to cause the damage shown in photos would almost certainly indicate an improper installation of gas lines or appliances, or a deliberate tampering with the installation (not necessarily with the intent to cause an explosion), but the air volume in a house the size of the surrounding ones shown in news photos would be more than sufficient to capture enough (leaked) gas to cause an explosion of the size shown.

"Famous gas explosions" in family history include the total destruction of the Trailways Maintenance Shop at Kip Kansas (while a backdoor relative worked there) that completely removed a shop large enough to accomodate almost a dozen Trailways buses for similtaneous maintenance, with the source being a single tank holding about only about 160 cubic feet of gas, but that was "acetylene" (ethylene) from a welding gas generator which is a lot more agressive than natural gas.

A natural gas leak at Pretty Prairie Kansas pretty completely destroyed at least five buildings on the main drag ca 1938(?) when the dry cleaning shop blew. The shop was about the same size as the 2-chair barber shop next door (<3,000 cubic feet?), so wasn't remotely as large as the size of houses shown in the Indy neighborhood.

John