(CBS) Top defense official suggests Russia violating nuclear test ban treaty. Quoting,

When pressed on the allegation by Journal reporter Michael Gordon, Ashley would only say Russia had the “capability” to conduct very low-yield nuclear tests, a capability which Russia, China and the United States have long possessed, according to the Arms Control Association. He did not say whether Russia has conducted or is conducting such tests.

In the early days, as with North Korea today, a test was conducted at the end of a zig-zag tunnel, extending  horizontally into a mountain. The zig-zags cause the tunnel to collapse with minimal leakage. At the end of a  tunnel branch is a test chamber. At a minimum, the chamber must hold the “gadget”, and the monitoring equipment.

Tunnel excavation is accomplished with mining machinery, or, as in North Korea, with slave labor. In the U.S., in the late 1950’s, tunnels were replaced by vertical boreholes, requiring far less manpower, and without  risk to life.

The test chamber of a borehole is the width of the hole itself, so a standard oil rig cannot be used. The U.S. has special large-diameter rigs for test boreholes, and prefabricated instrumentation packages to  fit. The diameter, on the order of 6 feet, allows no  extra room to cushion the seismologic signature of the explosion. Hence even very small explosions above zero yield can be distinguished.

At last inspection of the active-till-1989   Semipalatinsk Test Site in Kazakhstan , Soviet underground tests were still conducted in tunnels. Lacking specialized large bore drills, this remains the Russian approach today.

A chamber inside a mountain is not subject to a fixed size. It can be excavated to whatever size is required, subject only to the load-bearing capacity of the rock. A cavern of modest size can damp explosions in the range of bunker busters, around 100 tons.

For maximal disguise, the objective of the Russians would be to create a seismologic signature that cannot be compared to preexisting data in U.S. possession. By choice of absorbent fill material, the seismologic signature can be further diffused and disguised. Fill  avoids the signature of a cavern collapse. Once you’ve heard one of those, you’ve heard them all.

The inconclusive  evidence, which cannot be dismissed, likely consists of:

• Seismological signatures that, because of lack of comparison data, cannot be definitively classified.
• Other forms of intelligence about which I will not comment.