Undersea acoustic monitors set up to detect illegal nuclear activity pick up all sorts of other sounds as well, ranging from whales to earthquakes. (CTBTO)

It records sounds that no human ear can hear, like the low roar of a meteor slicing through the upper atmosphere, or the hum an iceberg makes when smacked by an ocean wave.

It has picked up threats invisible to the human eye, such as the haze of radioactive particles that circled the planet after the Fukushima nuclear accident in Japan in 2011.

The engineers who designed the world’s first truly planetary surveillance network two decades ago envisioned it as a way to detect illegal nuclear weapons tests. Today, the nearly completed International Monitoring System is proving adept at tasks its inventors never imagined. The system’s scores of listening stations continuously eavesdrop on Earth itself, offering clues about man-made and natural disasters as well as a window into some of nature’s most mysterious processes.

The Obama administration hopes the network’s capabilities will persuade a reluctant Senate to approve a nuclear test-ban treaty that stalled in Congress more than a decade ago. Meanwhile, without the treaty and wholly without fanfare, new stations come on line almost every month.

“We can pick up whale sounds, and ice sheets cracking,” said Thomas Muetzelburg, a spokesman for the Comprehensive Nuclear Test-Ban Treaty Organization (CTBTO), the Vienna-based group that operates the network. More importantly, he said, “we can reliably detect nuclear tests.”

The monitoring system is a latticework of sensors — including radiation detectors and machines that measure seismic activity or low-frequency sound waves — spread out across 89 countries as well as the oceans and polar regions. Like a giant stethoscope, it listens for irregularities in Earth’s natural rhythms, collecting and transmitting terabytes of data to a small office in the Austrian capital.

The network was designed to help enforce the 1996 Comprehensive Nuclear Test-Ban Treaty, which outlawed explosive testing of nuclear weapons. But while the treaty has never entered into force — the United States and seven other countries have declined to ratify it, in part because of concerns over verification — the monitoring network has steadily grown over the years, from a handful of stations in 2003 to more than 270.

The network has emerged as one of the most compelling arguments for the treaty, advocates say. Arms-control officials in the Obama administration have cited the network’s advances in arguing for a new push for Senate ratification of the nuclear test ban, despite opposition from prominent Republicans who argue that the pact undermines U.S. interests.

“There has been a growing realization, especially after Fukushima, that the International Monitoring System has improved to an impressive level,” Rose Gottemoeller, the State Department’s assistant secretary for arms control, verification and compliance, said in an interview. “It became clear that the time is right to go out and talk about these accomplishments and what the treaty can do for U.S. national security.”

The test-ban treaty has not come up for a vote in the Senate since 1999, when it was soundly defeated. The vote embarrassed the Clinton administration and drew criticism from countries around the world, including Russia, which is among the pact’s 161 signatories.

At the time, the United States had already halted testing of its nuclear weapons under an undeclared moratorium that began in 1992. Still, most Republicans and many Democrats opposed the treaty on the grounds that it offered false security; at the time, there was no global system in place that could reliably detect cheating.

The International Monitoring System was designed to address those concerns. Construction began on the first listening stations in 2000, and dozens were operating by 2006, when the network experienced its first major test — an underground detonation that signaled North Korea’s emergence as a nuclear weapons power. The country’s communist government would conduct two additional nuclear tests, in 2009 and last February.

“We detected the announced 2006 test even though only 60 percent of the network was in place,” said Muetzelburg, the ­CTBTO official. He said 21 of the system’s stations registered that event, compared with 94 seismology stations detecting North Korea’s test in 2013.

As it turned out, the network’s listening posts did more than simply pick up the tests’ underground shock waves. Numerous stations also picked up trace amounts of radioactive xenon, a gas emitted in nuclear explosions. From studying the xenon plume, scientists could even calculate how long North Korea waited to open the sealed entrances to its underground test chambers.

Arms-control experts say the North Korean tests show that the monitoring system is up to the challenge for which it was designed — deterring secret nuclear weapons tests. While today’s advanced computers can conduct simulations of a nuclear explosion, few countries would seek to field a nuclear arsenal — or modernize an existing one — without testing whether the technology works, according to a senior European diplomat involved in arms-control negotiations.

“This actually works in the favor of the United States, because it effectively freezes its nuclear advantages in place while inhibiting other countries from developing nuclear weapons,” said the official, who insisted on anonymity because he was not authorized to discuss the matter.

Arms-control advocates worry that a failure to approve the treaty will eventually result in increased competition among countries seeking to acquire nuclear weapons or upgrade their arsenals. If all hope for passage dies, support for the monitoring network — which costs more than $120 million annually in member-state dues to maintain — will probably dry up as well, proponents say, leading to the loss of a scientific asset with benefits to the entire planet.

In the past two years alone, advocates note, the network has contributed to scientists’ understanding of a variety of natural disturbances. When a deadly tsunami damaged Japan’s Fukushima nuclear reactor, the network’s sensors tracked the spread of airborne radioactive contamination across the Pacific Ocean to North America and beyond. And last year, when an asteroid exploded over Russia’s Chelyabinsk province, the system’s monitors were able to track the trajectory and size of the space rock as it plummeted toward Earth.

Current and former U.S. officials who have followed the network’s evolution say its capabilities are impressive, but they acknowledge that it may not be enough to sway doubters in Congress. While President Obama has promised to push for ratification of the test-ban treaty, former lawmakers say the public support necessary to win passage has not materialized.

“It looks pretty far away at this moment,” said former senator Richard G. Lugar (R-Ind.), a champion of arms control during his three decades in Congress who opposed the test-ban treaty in 1999, in part because of concerns over verification. “Just getting the START treaty across the finish line was such a struggle, and it was relatively modest.”

Gottemoeller, the State Department official, noted that support for the 1960s ban on aboveground nuclear tests was prompted by a global concern about radioactive contaminants in milk. “The Limited Test Ban Treaty grew out of a mother’s movement,” she said. “We need to generate that kind of public support.”