The largest single cell in the human body is the egg, which is roughly the size of the period at the end of this sentence. That’s not unusual–in even the most basic biology class, students learn that individual cells are tiny little things, viewable only under high magnification. But xenophyophores break the rules a bit. The single-celled organisms live exclusively in deep-sea habitats and reach sizes as large as 4-inches across.

First discovered in 1889, the marine protozoans were thought to be sponges. Further study and several reclassifications later, we know that the organism is a fragile single cell, of which we’ve identified around 40 species. Their extreme fragility makes studying them in laboratory conditions impossible–they’re invariably damaged during collection and transport–but what we do know suggests that the giant amoebas are integral to marine biodiversity, playing roles as both food and stirrers of sediment, which gives other deep-sea creatures habitable locations on the ocean floors.

This week, researchers from the Scripps Institution of Oceanography at UC San Diego working with National Geographic revealed that xenophyophores have been discovered in the Mariana Trench. The team used dropcams to record activity in the trench–more than 6 miles below sea level. Footage shows xenophyophores live abundantly in the deepest areas, much lower than the Abyssal Plains where they’re most frequently observed. As a bonus, the video also shows the deepest ever spotting of a jellyfish.

As we study the ocean depths further, it becomes clear that we know very little about how diverse and complex the life is there. And worse, we understand even less about our impact on the ecology in areas we almost never see but undoubtedly affect. “As one of very few taxa found exclusively in the deep sea, the xenophyophores are emblematic of what the deep sea offers. They are fascinating giants that are highly adapted to extreme conditions but at the same time are very fragile and poorly studied,” explains Lisa Levin, director of the Scripps Center for Marine Biodiversity and Conservation. “These and many other structurally important organisms in the deep sea need our stewardship as human activities move to deeper waters.”

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