A worker pushes a cart Wednesday in the tunnel beneath a pit at the Nevada Test Site where an explosives tests will take place in June. The tunnel will simulate a location where weapons of mass destructions could be buried. Photo by Clint Karlsen.

NEVADA TEST SITE -- Miners took a break Wednesday from drilling and blasting a large pit in which 700 tons of explosives is scheduled to be detonated June 2.

With the 36-foot-deep pit only two-thirds finished, work halted as Defense and Energy officials offered a tour of a tunnel 100 feet beneath the pit and assured reporters they can safely conduct the Divine Strake bunker-buster test if all goes as planned.

The massive detonation of ammonium nitrate and fuel oil set off by C-4 explosives will give weapons scientists data on how shock waves travel through a 100-foot-thick block of bedded limestone. The tunnel will offer evidence of the blast's power to destroy a buried cache of weapons of mass destruction.

The above-ground blast near the top of Syncline Ridge will send a mushroom-shaped dust cloud 10,000 feet into the atmosphere and release an explosive yield equivalent to detonating 593 tons of TNT, 85 miles northwest of Las Vegas, said officials with the Defense Threat Reduction Agency. That would be larger than the 430-ton yield produced by the Danny Boy nuclear bomb that was set off in a basalt crater at the test site in 1962.

The $23 million Divine Strake test will be the culmination of a decade of planning and experimentation aimed at fine-tuning confidence in the ability of existing weapons to defeat deeply buried, hardened targets.

One official, Doug Bruder, a civil engineer who leads the agency's Counter Weapons of Mass Destruction Directorate, denied that the Divine Strake test is geared to developing a new nuclear bunker-buster bomb as some independent scientists have speculated.

Instead, he said, the effort is to assess the capabilities of current weapons to penetrate a target through explosive shock waves in a specific geologic setting -- in this case, a limestone tunnel.

The test supports how officials can "best plan for those weapons to be used if we ever have to," Bruder said. "What it also gives us in the future is how high the bar needs to be in terms of our future advanced explosives."

Aside from the Divine Strake test, he said the agency has a large program to explore more powerful conventional explosives.

"We want those explosives to be as powerful as possible but non-nuclear. So we need to know what does it take to actually defeat a facility like that. Now we know what we actually have to achieve in terms of power of the new explosive," he said.

Since construction of the 1,100-foot-long tunnel was completed in 1999, the agency has conducted 45 tests, including live munitions dropped by Air Force warplanes, he said.

That is in addition to small-scale laboratory experiments for the project and a pair of medium-scale explosions at the Mitchell limestone quarry, about 35 miles south of Bloomington, Ind. Those tests in 2004 and 2005 were powered by 3,000 pounds of nitromethane.

Officials for the National Nuclear Security Administration, a branch of the Department of Energy that is hosting the test, would not comment on a lawsuit seeking to block the test that was filed last week by Western Shoshones and downwinders from Utah.

Nevada environmental officials meanwhile, have asked the NNSA for more information that demonstrates harmful pollutants won't be released beyond the boundary of the 1,375-square-mile test site.

Most above-ground contamination sites are more than four miles away from the tunnel. A muck pile from six nuclear tests that were conducted below ground is more than a mile away. Those below-ground, weapons effects tests were conducted between 1962 and 1971, NNSA officials said.

The atmospheric, atomic bomb tests -- four each in two locations -- were conducted during the 1950s.

During Wednesday's preview tour, Linda Cohn, an NNSA environmental protection specialist, offered assurances that no radioactive materials from past nuclear tests at the test site would be injected into the atmosphere and carried beyond the test site's boundary.

She said survey's conducted Tuesday confirmed that "there is no radioactive contamination adjacent to this experiment site."

"The crater from this test is only about 98 feet in radius. It will be a large cloud but it's not going to go off site," Cohn said.