Israel has just carried out a major aerial exercise, putting a hundred or so F-15s and F-16s into the skies over the eastern Mediterranean, evidently a rehearsal for a strike against Iran's nuclear facilities. The move follows the statement earlier this month by Shaul Mofaz, Israel's deputy prime minister, that an Israeli attack on Iran's nuclear program is "unavoidable." Israel almost certainly knows the location of some of the critical nodes in the Iranian program that it must hit if it is to set the Iranian effort back by several years. It also possesses the technology to assure that its bombs will fall close to or on their targets. But would such a strike succeed?

We cannot know the answer, and neither can the Israelis. The question calls attention to what might be called the ongoing Counterrevolution in Military Affairs.

The Revolution in Military Affairs was based upon silicon, in particular the computer chips that make for precision-guided weapons. In the 1980s, the United States developed the technology to drop munitions near enough to their targets to ensure a high chance of destruction. In World War II, the circular error probable--the radius of a circle into which a projectile will land at least 50 percent of the time--was more than half a mile. Today, thanks to GPS systems and laser- and infrared-guiding devices, the radius is less than two dozen feet. Almost any given target can be knocked out by the use of just one or two conventional bombs.

In the face of the threat of such efficient destruction, Iran has not stood still. Some of its countermeasures are themselves based upon computerized systems, including highly effective Russian-made surface-to-air missiles that Iran is set to take delivery of this fall. But Iran is also employing a far older means of warfare: deep burrowing.

Subterranean combat is familiar to all students of military affairs. During the Civil War, soldiers with coal mining experience dug a 511-foot-long tunnel some 50 feet beneath the Confederate lines at Petersburg, Virginia. The terminus was filled with 8,000 pounds of gunpowder, and the blast killed between 250 and 350 Confederate soldiers. (The operation ended in disaster, however, when the Union troops who had rushed into the crater to follow up the attack were slaughtered by Confederate troops firing downward from the rim in what was described as a "turkey shoot.")

More typical, though, is defensive digging. In Berlin, beneath an otherwise unremarkable Chinese restaurant, are the ruins of the most notorious underground facility in history: the Führer bunker. Adolf Hitler held court here in the last phase of World War II, and it was in the bunker that, on April 30, 1945, together with his new bride, he ingested cyanide. As an engineering feat, the Führer bunker was not particularly impressive; Hitler's honeymoon grave was a mere 28 feet underground.

At the dawn of the nuclear age, the USSR constructed a vast network of tunnels under Moscow, including a 17-mile secret subway line to Vnukovo airport, to ensure that the leaders of the Kremlin would survive a nuclear strike. Some of these underground facilities were hundreds of yards deep and could accommodate thousands of people, sustaining them in compartments impervious to chemical and biological attack. It required a totalitarian system to marshal the manpower and resources to remove such an immense quantity of soil and rock.

Today, however, tunneling is far cheaper and easier. In the early 1990s, the Chunnel, the 30-mile rail tunnel connecting France and England, was built using drilling machines that hewed out a 30-foot diameter circle of rock at the remarkable pace of 164 feet a day. Modern drills are huge, multi-million-dollar pieces of machinery. They operate with a circular disk on the front end that holds steel teeth, which cut into the rock as the plate rotates. A conveyor system pulls the spoil backward, while workers follow up, erecting a reinforced lining for the excavated structure. Narrower diameter tunnels than the Chunnel can be carved into solid rock at the staggering rate of 650 feet per day.