Corals reefs can stretch for thousands of kilometres, and the photogenic menagerie of animals they house is obvious to the naked eye. But reefs are built by tiny coral polyps—tentacled animals that look like sea anemones and are just millimetres in size.

The microbes they depend on, including the algae that provide them with energy and the bacteria and viruses that smother their surfaces, are smaller still. To understand how reefs really work, you need to look closer.

So Andrew Mullen and Tali Treibitz from the University of California San Diego have created an underwater microscope that, for the first time, allows divers to zoom in on wild corals. They call it the Benthic Underwater Microscope, or BUM.

Using the