In high-rise offices along the Danube, scientists riveted to computer screens "listen" to sounds no one can hear, "feel" every rumble in the Earth, "sniff" global skies for exotic gases _ on alert for signs of a newborn atomic bomb.

Governments over the past decade have quietly built up a $1 billion International Monitoring System to enforce the treaty banning nuclear weapons tests. At more than 200 stations around the world, from deep in the Pacific to high in the Bavarian Alps, they have deployed advanced technologies to detect secret explosions. And they have waited.

Since 1999, when a Republican-led U.S. Senate rejected it, the treaty has languished in a diplomatic limbo, and this unequaled _ and growing _ system of global sensors has remained in long-running rehearsal.

Barack Obama wants to change that.

"After more than five decades of talks, it is time for the testing of nuclear weapons to finally be banned," the U.S. president said in a pivotal speech April 5 in Prague, Czech Republic.

Major nuclear powers, including the U.S., have observed moratoriums on testing since the 1990s, but India, Pakistan and North Korea all have tested bombs since the Comprehensive Test Ban Treaty was negotiated.

Obama vowed to "immediately and aggressively" pursue treaty ratification by the Senate, now in Democratic hands. If other holdout countries follow suit, the "CTBT" would come into force, putting the power of international law and the U.N. Security Council behind a ban.

A new report from a divided U.S. congressional commission, however, signals that the debate will be a difficult one _ between those who see a test ban as a step toward de-nuclearizing the world, and those who see it as a risk for U.S. national security. And that debate will focus on how verifiable the treaty is, on just how good a global alarm system all that money has bought.

The French engineer in charge of completing it says it's very good.

"We can already see the network is providing much better performance than envisaged at the time the treaty was negotiated," Patrick Grenard said. "It's extremely sensitive."

Three-quarters of the planned 320 stations are built, certified and on line, each using one of the system's four technologies: seismic, sensing the shock waves of an underground blast; hydroacoustic, listening for underwater explosions; infrasound, picking up the low-frequency sound of an atmospheric test; and radionuclide detection, sampling the air for a test's radioactive byproducts.

From the Arctic to Antarctica, from dozens of islands in the world's oceans, from forests, mountaintops and cities on every continent, the stations transmit data via six satellites back to the Vienna headquarters building of the Comprehensive Test Ban Treaty Organization.

Seismologists, physicists and other specialists among the agency's 286-member staff review the stream of information _ readings of earth tremors and mining explosions, of undersea volcanos and discharges from nuclear power plants. They then package the data and relay it to the treaty's signatory nations, including the U.S., which signed the pact in 1996, only for the Senate to reject it three years later.

While not yet fully accepting the treaty, the U.S. government benefits by obtaining data from monitoring stations in China, Russia and other sensitive places, even Iran. In fact, the U.S. itself hosts more stations than any nation _ 38 when the network is completed _ and pays 22 percent of the treaty agency's operating costs.

Installations at tiny Wake Island, a U.S. territory in the remote mid-Pacific, typify America's commitment to the infrastructure of a treaty it isn't fully committed to.

In 2006-2007, the Vienna agency and the U.S. Air Force built an $18 million hydroacoustic facility at Wake, the costliest single station in the global system. Three hydrophones in globular nodes were moored to seamounts in each of two locations about 100 kilometers (60 miles) from the coral atoll's shores, linked to Wake by undersea cable.

Some 750 meters (2,500 feet) down, the listening devices take advantage of a layer of ocean that, because of temperature and salinity, "traps" and transmits sounds over vast distances.

"Blast fishing" _ use of a couple of kilograms (pounds) of dynamite to kill fish _ "can be heard 2,000 kilometers (1,200 miles) away," said Andrew Forbes, a British hydroacoustic specialist here.

Wake rounded out the "hydro" network of 11 stations, enabling the Vienna monitors to listen in to all the oceans, confident of detecting an explosion down to the unlikely micro-level of one ton of TNT equivalent, 15,000 times smaller than 1945's Hiroshima bomb.

Above ground, plans call for Wake also to host an infrasound station, a sprawling array of microbarometers that detect minuscule changes in air pressure _ in effect, "hearing" atmospheric vibrations at frequencies below the human ear's 20-hertz minimum.

The sound of an aboveground blast dies quickly in the atmosphere, but its "infrasound" can circle the globe, to be picked up in Australia, the Azores or any of 58 other locations.

Because of its strategic location, Wake also has a radionuclide station, sampling air currents for strontium-90 and other radioactive fallout, smoking guns of a nuclear explosion. Half the system's 80 radionuclide stations, including Wake's, are also being equipped with gear to detect gases such as xenon and krypton, which are created in nuclear blasts.

Fallout from underground tests usually is absorbed in surrounding rock, but the gases seep out. Xenon was the smoking gun in North Korea's nuclear test.

Shock waves from that October 2006 explosion registered at half the monitoring system's 40 operating seismic stations. Then an agency radionuclide post in northern Canada detected trace amounts of xenon-133 in the air. Computerized wind models enabled the Vienna team to track the gas back to North Korea, confirming the explosion was nuclear.

The monitoring system is operating on a provisional basis. If the treaty enters force, it authorizes onsite inspections of suspicious events. Together that "will enable us to detect any possible test which is militarily significant," Hungary's Tibor Toth, the treaty organization's executive secretary, said in an interview.

That statement may be challenged on the U.S. Senate floor.

On May 6, a congressional commission on nuclear policy, led by two former defense secretaries, William J. Perry and James R. Schlesinger, issued a final report in which one faction asserted the test-ban treaty is "wholly unverifiable" for very low-yield, clandestine underground tests. The other faction said such concerns were "overstated."

As in 1999, treaty opponents are expected to contend that "decoupling" a small blast, conducting it in an underground cavity so large its shock waves are muffled, will defeat the seismic monitors.

But a 2002 U.S. National Academy of Sciences study listed 10 logistical difficulties a nuclear newcomer would face in attempting such decoupling. Another study by U.S. government advisers said a very small test _ in the range of 500 tons TNT equivalent _ would produce little of military significance. Besides, said physicist David Hafemeister, a 2002 study participant, the latest satellite radar technology would spot even a slight depression in the earth caused by an underground blast.

Next month in Vienna, the treaty organization will assemble scores of scientists from around the world to assess the International Monitoring System. Their endorsement would be a boost for treaty supporters. But a new, all-American assessment might prove more important to a Senate debate.

Geophysicist Raymond Jeanloz, chairman of the National Academy's Committee on National Security and Arms Control, favors conducting a new study of a system now nearing completion.

"Very reasonably, a political leader might ask, 'Is it performing up to standards?'" he said. "The scientific community is in a position to give an answer."

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EDITOR'S NOTE _ Charles J. Hanley has been reporting on nuclear arms control since 1983.