Spy satellites sweep the planet with cameras each day. Sensitive instruments can pinpoint even tiny tremors around the globe. So if you're a renegade nation with a small atomic bomb to test, where can you do it in privacy?
Lynn R. Sykes, a professor at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y., said yesterday there was really only one practical covert nuclear testing ground left: huge caverns, called "salt domes," located up to 6,600 feet underground.
Dr. Sykes, speaking at the American Geophysical Union meeting in Baltimore, said Russian scientists disclosed that the former Soviet Union conducted such a test on March 29, 1976, just east of the city of Azgir in Kazakhstan.
A dome created by an earlier, much larger underground test was used to muffle the burst of a bomb with the power of 8,000 tons of TNT, or 8 kilotons.
Such covert testing is expected to be the key concern of negotiators working on a comprehensive nuclear test ban treaty, Dr. Sykes said. Congress has pledged to move the United States toward such a treaty by 1986.
Salt domes quiet the blast by evenly absorbing its shock wave, preventing the wave from setting off tremors in the surrounding rock. The salt also seals in telltale radioactive gases that might be detected by downwind instruments.
Most bombs in the arsenals of the United States and the former Soviet Union are huge, with the explosive power of hundreds of thousands or millions of tons of dynamite. These are easy to detect, because they set off the same level of shaking as a respectable-size earthquake.
But small bombs -- with the power of 10 kilotons or less -- can be muffled.
To bottle up even a small nuclear explosion, the salt dome must be very big. For a 5-kiloton blast, Dr. Sykes said, a cavern would have to be taller than the Statue of Liberty.
The good news, Dr. Sykes said, is that the locations of existing domes, both natural and artificial, are well known. Building networks of seismic monitoring stations nearby, he said, could allow detection of blasts from all but the tiniest nuclear devices.
In the former Soviet Union, Dr. Sykes said, the United States should ask to install monitors in a region north of the Caspian Sea containing parts of Kazakhstan and Russia.
Similar networks could be set up, he said, in other regions of the globe, with Pakistan monitoring India, for example, or with Middle Eastern states monitoring each other.
Dr. Sykes warned against trying to build an expensive network of seismic stations across the former Soviet Union. Such a network would probably wind up detecting nearly 50,000 chemical explosions and small earthquakes each year.