has a very high resistance and acted like an electrical fuse, basically melting. two houses next door to each other, both with gas meters on fire, unusual circumstances to be sure. but from there, it gets even more unusual from a fire investigator's point of view. >> the fires developed in such a way that the two small fires that are being dealt with. and then within a few minutes, the third house up, the door is slightly open. and all of a sudden, you can see the front door slam into the door frame. this all happens in less than a second. >> out of nowhere, this third house that seemingly has nothing to do with the fire two fires explodes. clark's investigation reveals the explosion is caused by a back draft. >> so within a split second, the