It sounds like a subway train rushing by. Or a plane flying low overhead. But it's something much more exotic: In all likelihood, the first "marsquake" ever recorded by humans.
NASA's InSight mission detected the quake April 6, four months after the lander's highly sensitive seismometer was installed on the Martian surface.
Since then, the instrument has registered the howling winds on the red planet and the motions of the lander's robotic arm. But the shaking picked up this month is believed to be the first quake detected rumbling through Mars' interior.
"We've been collecting background noise up until now, but this first event officially kicks off a new field: Martian seismology!" Bruce Banerdt, InSight's principal investigator, said in a statement. Banerdt studies planetary seismology at the Jet Propulsion Laboratory in La Canada-Flintridge, Calif.
For many, it's been an agonizing wait.
"We all knew it was just a matter of time," said Renee Weber, a planetary scientist at NASA's Marshall Space Flight Center in Huntsville, Ala., and a member of InSight's science team.
Collaborators in Europe are the first to see the data when it's beamed down from the spacecraft, so Weber knew the news of a quake would come in the middle of the night for U.S. researchers.
"I always checked first thing before I get out of bed," she said in an interview. "Is today going to be the day?"
Researchers were elated when the quake finally happened.
"It's so exciting to finally have proof that Mars is still seismically active," Philippe Lognonne, a geophysicist at the Institut de Physique du Globe de Paris in France, said in a statement. Lognonne is the principal investigator for InSight's Seismic Experiment for Interior Structure, the official name for the seismometer.