have a yield as low as a kilogram, up to low tons. Hydronuclear tests do involve nuclear reactions, but very small ones. A technique that actually may have more explosive yield, of high explosive, is hydrodynamic testing, in which extremely fast X-ray, neutron, or other specialized camera measure, in microseconds, the explosive compression of a fissionable material simulant. Depleted uranium, for example, has the same physical properties as enriched uranium, and is similar to plutonium. Acoustic Intelligence Sensors relatively close to a nuclear event, or a high-explosive test simulating a nuclear event, can detect, using acoustic methods, the pressure produced by the blast. These include infrasound microbarographs (acoustic pressure sensors) that detect very low-frequency sound waves in the atmosphere produced by natural and man-made events. Closely related to the microbarographs, but detecting pressure waves in water, are hydro-acoustic sensors, both underwater microphones and specialized seismic sensors that detect the motion of islands. On-site Inspection The US and Russia have agreed to have, under controlled conditions, inspectors from the other side physically examine locations at which a forbidden nuclear test, possibly below other detection thresholds, may have taken place. In the United States, these programs are operated by the Defense Threat Reduction Agency, which superseded the On-Site Inspection Agency. While inspection procedures as specific as those for nuclear weapons have not been developed for chemical and biological threats, on-site inspection is likely to be needed, as far more chemical and biological manufacturing processes have dual-use properties: they can be employed for perfectly legitimate civilian purposes. The Director of DTRA is also "dual-hatted" as the head of the Center for Combating Weapons of Mass Destruction (SCC WMD), an agency of the US Department of Defense's Strategic Command. This mission also relates to that of the CIA Counterproliferation Center. Materials intelligence and Air Sampling Nuclear tests, including underground tests that vent into the atmosphere, produce fallout that not only indicates that a nuclear event has taken place, but, through radiochemical analysis of radionuclides in the fallout, characterize the technology and source of the device. For example, a pure fission device will have different fallout products from a boosted fission device, which, in turn, differ from various types of thermonuclear devices. One real-world example is a review of how xenon by-product levels could be used to distinguish if air sampling from a North Korean test, either atmospheric testing or leakage from an underground test,