The deepest undersea eruption on record has been discovered, given away by striking glassy lava formations on the Pacific Ocean floor.
The eruption took place 4.5km beneath the surface of the ocean in the Mariana Trough, south of Japan, somewhere between February 2013 and December 2015, scientists say.
It was discovered in December 2015 by an autonomous submarine looking at thermal vents at the bottom of the ocean, and revealed on Friday in journal Frontiers in Earth Science.
"We know that most of the world's volcanic activity actually takes place in the ocean, but most of it goes undetected and unseen," said marine geologist Bill Chadwick, who led the study.
"Many of these areas are deep and don't leave any clues on the surface. That makes submarine eruptions very elusive."
At the beginning of the 1990s, no undersea eruptions had ever been found, he said - there have now been 40, including a monster one off the coast of New Zealand in 2012.
The lava flows spotted by the unmanned submarine stretched more than 7km across the ocean floor, and were up to 138m thick.
"Undersea volcanoes can help inform us about how terrestrial volcanoes work and how they impact ocean chemistry, which can significantly affect local ecosystems, said Prof Chadwick.
"It's a special learning opportunity when we're able to find them."
In 2012, an undersea eruption was spotted by an artist on board a plane flying from Samoa to New Zealand.
"I took a couple of pictures, wondering if it was an algal bloom, oil spill or, recalling a conversation with a friend the week before, a deposit from a volcano," artist Maggie de Grauw told the Waikato Times in 2013.
A two-year study into the Havre Volcano eruption found it was potentially the biggest in the past 100 years - but as it took place at the bottom of the ocean, it almost went completely unnoticed.