The blast shook the ground, and authorities said it was caused by just a one-third pound of explosives.
Authorities said 250 pounds of explosives were stolen sometime between September 28 and October 7 last year from a locked container on a remote hillside.
The explosives were being stored by a man whose father worked in commercial explosives many years ago. Authorities said they found 100 pounds of the explosives on site, and they destroyed them immediately because it was just too unstable, but 150 pounds are still missing. They are stored in five 50-pound boxes.
Three boxes of electric blasting caps are also missing.
"The balance of the unrecovered explosives is likely to have been stashed or dumped somewhere in the Black Star Canyon area or the Cleveland National Forest," said Capt. Mark Billing from the Orange County Sheriff's Department.
A $25,000 reward is being offered for information that leads to the arrest and prosecution of those responsible.
Anyone with information is urged to call ATF at (888) ATF-BOMB.
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