Bagram, Afghanistan ? U.S. troops have destroyed the largest cache of explosives yet found in Afghanistan hundreds of 500-pound bombs buried in a dry riverbed near Kandahar, the military said Friday.

The cache of 420 air-to-ground bombs was found last month in the Dori River channel, but it took munitions experts weeks to examine the explosives and decide how to deal with them, Air Force Maj. Steve Clutter said.

After evacuating residents, demolition teams used 30,000 pounds of C-4 explosive to destroy the bombs in a single blast Thursday. Army photos of the blast showed a huge mushroom cloud rising over the site.

“It was a big bang. It was probably like the Fourth of July down there,” Clutter said at Bagram Air Base, the U.S. military’s headquarters in Afghanistan.

There were enough bombs hidden in the riverbed to fill six B-52 bombers, Clutter said.

He was unsure how the troops discovered the bombs, the age of the explosives or when they were buried.

“It is an example of how much work we have yet to do,” said Defense Department spokeswoman Victoria Clarke in Washington.

“We continue to find caches of weapons and ammunition. Clearly the al-Qaida, the Taliban has a great deal of intent some still have intent to do harm.”

Explosives experts decided the bombs were not stable enough to move and decided to string them together with explosives to destroy them all at once, Army Maj. Richard Patterson said.

An Army ammunition expert prepares explosive charges to 500-pound bombs found buried in a dry riverbed near Kandahar, Afghanistan. All 420 bombs the largest cache found by U.S. troops in Afghanistan were blown up Thursday in a single explosion.

Troops met with village elders to warn them about the demolition, and an aircraft was sent to patrol the area around the blast, he said.

The bombs each contained about 250 pounds of shrapnel and 250 pounds of explosive enough to send deadly fragments about a third of a mile. The bombs were about 1 1/2 feet wide and about 6 feet long, U.S. explosives experts said.

Many weapons found in Afghanistan are Russian munitions left over from the 1980s. The ousted Taliban regime had a small air force, including 10 Russian-made Su-22 fighter-bombers, but most were destroyed by the U.S.-led military coalition last year.

U.S. forces searching Afghanistan for al-Qaida and Taliban leaders continue to turn up dozens of weapons caches each month.

The discoveries are either destroyed or handed over to Afghan officials to be given to the fledgling Afghan National Army.

U.S. officials are worried that large bombs like the ones destroyed Thursday could be used in booby traps and car bombs aimed at international forces or the government of Afghan President Hamid Karzai.