In 2015, an international team of researchers sent robotic submersibles beneath the waves north of Guam. They had set out to study an area south and west of the Mariana Trench — the deepest groove in Earth’s oceans — and an arc of volcanoes, hoping to spy hidden hydrothermal vents.

Instead, they discovered a spectacular glassy labyrinth, nearly three miles below sea level. It was recently cooled lava, the product of the deepest underwater volcanic eruption ever recorded by scientists.

The researchers reported their discovery last week in Frontiers in Earth Science. The identification of deep-sea eruptions happens very rarely, said Bill Chadwick, a seafloor geologist at the NOAA Pacific Marine Environmental Laboratory in Oregon and lead author of the new study, and discovering one “is an opportunity to learn about a fundamental Earth process that we know little about.”

The finding wasn’t just notable for its extraordinary depth. The extremely young age of the lava deposit offers scientists a window into the very beginnings of what happens when a volcanic outburst occurs beneath the seas. So often, they just see an epilogue.