Last November, the largest offshore volcanic event in recorded history happened. Seems like something you would have felt, right? Or at least heard of?

The thing is, this volcanic event was mysterious right from the start. It wasn’t initially felt by humans, since it happened deep beneath the Indian Ocean off the coast of Madagascar. But geologists knew something strange was going on.

For one, the seismic waves the blast gave off were super low and were only at a single frequency. Typically, seismic waves would operate on several frequencies. Plus, the nearby French island of Mayotte started experiencing some weirdness. Right after the event, it moved a few inches. Then, it started experiencing more than 1,000 tiny earthquakes each day.

It was enough that an international team of scientists started to dig into what happened last November, and what they found was incredible: They believe the massive seismic event birthed a giant underwater volcano.

As fantastical as they sound, underwater volcanoes are a pretty common phenomenon. It’s tough to know just how many, since Earth’s oceans are alarmingly unexplored – they cover more than 70% of the planet’s surface and ultimately provide us with the resources we need to survive, and yet 80% of those oceans are currently unmapped, unobserved and unexplored.

But many scientists try to estimate what’s lurking beneath those unexplored waters, and some believe that there are as many as 1 million submarine volcanoes scattered across the world’s sea floors. There could be as many as 75,000 with peaks more than an entire mile high.

Most are too far underwater to impact the daily lives of humans on dry land. But not all! Kolumbo is an active submarine volcano off the coast of the popular Greek island of Santorini. It exploded in 1650, and the volcanic ash, lava and gases that the explosion created killed about 70 people.

So it’s no surprise that the volcano exists in the first place. What’s remarkable is that something so giant formed in only six months.

Imagine you’re in your backyard on a sunny July day and, out of nowhere, there’s a sudden seismic event and a volcano begins to form. You survive the event, and for the next six months, you feel more earthquakes as the volcano grows bigger and bigger. By the time Christmas rolls around, your backyard isn’t your backyard anymore. It’s the center of a volcano that extends three miles past your house and rises almost half a mile above it.

Sounds unbelievable, right? But that could be what happened deep within the Indian Ocean last November. Of course, this kind of stuff doesn’t happen in people’s backyards. But scientists are looking forward to discovering more about how this happens underwater, and what exactly went down in November. What they learn could unlock even more secrets about the mysteries deep inside the ocean.