WASHINGTON — The Pentagon plans to explode a massive 700-ton conventional bomb on June 2 at the Nevada atomic test site as part of a U.S. military program to develop weapons for destroying underground enemy bunkers.

The blast will be one of the largest explosive tests since the end of the Cold War.

"It is the first time in Nevada that you'll see a mushroom cloud . . . since we stopped testing nuclear weapons," James Tegnelia, head of the Pentagon's Defense Threat Reduction Agency, told reporters Thursday.

Tegnelia described the ground-level blast — dubbed "Divine Strake" — as an experiment to assess so-called bunker-buster weapons currently in development. The test "allows us to be able to predict . . . how well they can work against granite, hard structures."

A strake is a strip of planking along the side of a boat or a device on an aircraft's fuselage for controlling air flow.

Defense Intelligence Agency and Central Intelligence Agency officials have testified that potential adversaries such as North Korea and Iran increasingly burrow their secret weapons and command centers into well-protected underground bunkers.

The June explosion "represents to us the largest single explosive that we could imagine doing conventionally to solve that problem" of destroying underground bunkers, Tegnelia said.

The huge explosion 85 miles northwest of Las Vegas at the Department of Energy's remote Nevada Test Site will be one of the largest since the U.S. halted nuclear bomb tests in 1992.

Darwin Morgan, a spokesman for the facility, said the test will be the third such experiment there since nuclear testing was halted in 1992. In 1993 the government exploded a bomb composed comprised of 1,450 tons of conventional explosives in an underground test and, in 2002, exploded an 18-ton bomb there, he said.

The Nevada Test Site occupies 880,000 acres in southern Nevada, one of the largest restricted areas in the country.

Irene Smith, a spokeswoman for the Defense Threat Reduction Agency, said it will register between 3.1 and 3.4 on the Richter sale but that "there will be no adverse effects to surrounding facilities either on or off the Nevada Test Site."

The U.S. plans to alert the Russian government beforehand to explain that the seismic activity is not a nuclear test, Tegnelia said.

The U.S. atomic bomb that destroyed Nagasaki, Japan, in 1945 had an explosive yield equal to 22,000 tons of conventional explosives.

Any mushroom cloud kicked into the atmosphere by the June explosion would be free of radioactive debris because the bomb isn't a nuclear weapon and the ground where the explosion will occur was never used for atomic explosions, Smith added.

During the Cold War, testing of such massive conventional bombs was more commonplace. In tests at the White Sands Missile Range in New Mexico, for example, the military exploded 4,744 tons of explosives in 1985, 4,685 tons in a 1987 test, 2,445 tons in 1989, 2,440 tons in a 1991 test and 2,250 tons in 1981.

"Experience obtained from the detonations listed above," the Department of Energy said in an environmental impact statement for the forthcoming explosion, "was used to develop the plans for the proposed large-scale, open-air explosive detonation, Divine Strake, at the Nevada Test Site."