In the North Sea — between the United Kingdom, Norway and Denmark — thousands of flies swarmed an oil rig. Engineer Craig Hannah noticed they'd stay still on the rig for hours, suddenly taking off all at once.

They're called hoverflies. Often confused with bees, they're unsung pollinators. And they migrate, often hundreds of miles – including, it seems, to the middle of the ocean.

Hannah was stumped by the mystery. But, he thought scientists might be interested in studying these insects in the open ocean, since most bugs are studied from land. So he started collecting samples whenever groups of these hoverflies landed, and then sent them to a research team at the University of Exeter in the United Kingdom.

Eva Jimenez-Guri , a biologist on the team, says they were surprised by how much

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