Weird Hum Heard Around the World Was the Birth Of A Submarine Volcano

One of the stories on the Mayotte volcano referred to it as rare. They aren't rare at all.
Here's a 2007 story from NewScientist:

The true extent to which the ocean bed is dotted with volcanoes has been revealed by researchers who have counted 201,055 underwater cones. This is over 10 times more than have been found before.

The team estimates that in total there could be about 3 million submarine volcanoes, 39,000 of which rise more than 1000 metres over the sea bed....

Human beings aren't near as smart as we think we are, a point I exemplify on a daily basis at Climateer Investing.
First up, Stuff New Zealand, January 10:
Kiwi's interest in earthquakes helped scientists discover enormous volcano

The volcano came into the world wailing, but for a while nobody heard it - except for a man in Wellington.

It was born in the summer of 2018 just off the coast of the tiny island of Mayotte, a French territory halfway between Madagascar and Mozambique.

An earthquake swarm in May of that year precipitated its arrival like a drum roll. Magma rumbled from within a reservoir at the top of the Earth's mantle. The magma migrated up through the crust, sending tremors across the nearby island as it moved - until finally, sometime in late June or early July, with no precise birth date yet recorded, it popped its head out of the ocean floor.

For months, the underwater volcano announced its own birth with mysterious cries: a low seismic humming too faint to feel. It wasn't until November 11, 2018, that anyone noticed. Something strange happened that day. The seismic waves travelled all over the world, to Kenya and Chile, Canada and Hawaii, nearly 11,000 miles away. And the humming got louder, longer, lasting up to a half-hour.

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And from PhysOrg, January 6:

Formation of a huge underwater volcano offshore the Comoros

A new submarine volcano was formed off the island of Mayotte in the Indian Ocean in 2018. This was shown by an oceanographic campaign in May 2019. Now, an international team led by the scientist Simone Cesca from the German GeoForschungsZentrum GFZ is illuminating the processes deep inside the Earth before and during the formation of the new volcano. It is akin to detecting a new type of signal from the Earth's interior that indicates a dramatic movement of molten rocks before the eruption. With their specially developed seismological methods, the researchers are reconstructing the partial emptying of one of the deepest and largest active magma reservoirs ever discovered in the upper mantle. The study is published in the journal Nature Geoscience....

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There's another submarine volcano off the coast of Alaska that emits the infrasound
From LiveScience, October 17, 2019
Underwater Volcano Creates Bubbles More Than a Quarter-Mile Across

In 2018 we had "Ships in Caribbean Told to Avoid Underwater Volcano ‘Kick ’em Jenny’ Over Risk of Eruption" 
Avoiding anything named Kick 'em Jenny sounds like a good idea.

Six more were discovered off Italy last summer. Damn things are everywhere.