Two California families are lucky to be alive -- after a "Mythbusters" experiment went horribly awry yesterday ... sending a cannonball careening through their homes, leaving MASSIVE holes in its wake.

Sgt. JD Nelson from the Alameda County Sheriff's Department tells ABC, hosts Jamie Hyneman and Adam Savage were trying to figure out how effective a cannon is at firing non-cannonball objects ... but first, they had to test the cannonball itself.

The cannonball was supposed to land harmlessly in a nearby hill -- but something went wrong ... and the ball,  traveling at about 1000 ft/s, went over the hill bordering Camp Parks Military Firing Reservation, and into the neighboring town of Dublin, crashing through two separate houses before finally coming to rest ... inside someone's minivan.

The Discovery Channel show has been put on hold after the mishap, pending a full investigation. Luckily, no one was injured.

UPDATE: The show has not been put on hold according to star Adam Savage, who Tweeted the following on Thursday morning:

"Tweeps: thanks SO much for all the kind support. FYI, Mythbusters has NOT be suspended/cancelled. We're still in production. Happy holidays."