The Air Force is working feverishly, and secretly, on a nuclear warhead that will burrow deep into the earth and make mincemeat out of Soviet underground missile silos and control bunkers.

We have learned that both Lockheed and General Electric have been working on more than $5 million in Air Force contracts to produce prototypes of these earth-penetrating warheads by mid-1988. The Department of Energy, which has responsibility for production of nuclear weapons, has a piece of the research on this project, too.

The United States already has nuclear warheads that have a slim chance of taking out the strongest Soviet missile silo with a hit on the surface above the silo. The new burrowing warhead, instead of bursting in the air over its target, will cozy up next to the silo deep in the ground and explode at a certain, preset depth.

American nuclear weapons designers believe the new warheads could be fitted on U.S. weapons by the mid-1990s.

Pentagon strategists have long figured it would take two U.S. nuclear warheads to take out each Soviet silo. These would have to be warheads with "hard-target capability," meaning they could destroy underground missile silos.

A highly classified report by the Arms Control and Disarmament Agency notes that the largest force of U.S. missiles able to destroy hard targets are the upgraded Minuteman IIIs. Before the upgrade, they were topped with three W62 warheads, each capable of a 170-kiloton blast and suitable only, according to the report, to hit "soft urban industrial and military targets," but not Soviet missile silos.

U.S. military leaders decided to upgrade the Minuteman IIIs with the new W78 warhead, which began production in 1980. With their 335-kiloton wallop, more than 1,000 of these warheads have been produced and mounted on Minuteman IIIs along with a new guidance system increasing the accuracy of the missiles.

The secret ACDA report says that with the new W78 warheads, a single Minuteman III missile has a 76 percent chance of destroying a Soviet silo hardened to withstand pressure of 600 pounds per square inch. The previous system, with W62 warheads, had only a 51 percent chance. The updated Minuteman III has a 35 percent chance of destroying an ultra-hard Soviet silo built to withstand 5,300 pounds of pressure per square inch. The old system's chance against the ultra-hard silos is 15 percent.

The U.S. Navy has separately developed a missile, the D5, for use in the Trident II submarine-launched missile system with "dial-a-blast" capability from seven to 335 kilotons.

Deployment of MX missiles with a 300- to 500-kiloton blast "would give the U.S. intercontinental ballistic missile force a much improved time-urgent, hard-target kill capability," the ACDA report says.

The earth-burrowing warhead would be a monumental improvement over the Minuteman III, the D5 and the MX because it would explode underground, close to the silos and command centers, maximizing the chances of success.