(CNN) Mars is full of subtle sounds and thanks to NASA's InSight mission, we're finally able to hear them.
The stationary probe's seismometer, the Seismic Experiment for Interior Structure, is sensitive enough to pick up the most gentle of vibrations.
The seismometer has been listening out for earthquakes on Mars. Seismic activity can paint a picture of the interior of a planet and how it was formed, which is one of InSight's main objectives for Mars.
InSight landed on Mars in November 2018 and placed the seismometer on the Martian surface in February. But the Red Planet didn't produce any sounds until April.
And even then, something strange happened.
It's a seismic signal that was recorded April 6, and by all indications, the InSight team believes this sound is a quake from within the planet rather than something on the surface. But the signal was at a high frequency and nothing like it has been produced since.
InSight has detected more than 100 events since April and the researchers estimate 21 of them could be quakes.
NASA shared some recordings of rumblings captured in May and July. Both occurred below the human range of hearing, but were processed so we could hear them. The May quake had a magnitude of 3.7 and the July quake reached 3.3
Mars doesn't have tectonic plates, which are the cause of quakes on Earth. Martian quakes are caused by cooling and contraction, which create stress fractures on the crust.
Mars has a cratered surface, which allows quakes to persist for about a minute. On Earth, quakes last for seconds at a time.
The researchers have been learning to filter out other sounds that the sensitive seismometer picks up.
In December, InSight picked up the sound of wind on Mars.
"It's been exciting, especially in the beginning, hearing the first vibrations from the lander," said Constantinos Charalambous, an InSight science team member at Imperial College London. "You're imagining what's really happening on Mars as InSight sits on the open landscape."
When the InSight arm moves, friction can be heard in the seismometer, producing intriguing sounds.
Wind picks up during the day, so it's easier for the researchers to listen out for quakes at night.
But nightfall comes with its own unique sounds. The InSight researchers call them "dinks and donks" -- the expansion and contraction of parts inside the seismometer, possibly due to heat loss.
In one of the sound samples, a strange whistling can also be heard. The researchers think there is interference with the electronics in the seismometer.
InSight, or Interior Exploration using Seismic Investigations, Geodesy and Heat Transport, is a two-year mission to explore a part of Mars that we know the least about: its deep interior.
The suite of geophysical instruments on InSight sounds like a doctor's bag, giving Mars its first "checkup" since the planet formed. Together, those instruments take measurements of Mars' vital signs, like its pulse, temperature and reflexes -- which translate to internal activity like seismology and the planet's wobble as the sun and its moons tug on Mars.