A hidden world is fundamentally different from the undiscovered. We know the hidden world is there. We just can’t see it or reach it. Something about this tantalizing proximity has fascinated us throughout history. The blank spaces on the map that Joseph Conrad referred to in Heart of Darkness were, to Europeans, hidden worlds. The moon was (and is again) perhaps the best example of a hidden world—extremely visible to us, but inaccessible and mysterious. 

Hidden worlds exist in the great depths of the ocean and high above us in the planets of the night sky. But they are also all around us in the form of waves and matter and microbes, nocturnal animals and clandestine passages, waiting to be exposed. 

Technology has long played the spoiler to these worlds in hiding. We have used ships, airplanes, and rockets to shrink distances. Telescopes, cameras, satellites, drones, and radar help us peer into and map the places we cannot go ourselves. Advances in computer vision and artificial intelligence are creating accessible replicas of complete physical worlds in virtual spaces. All this peels away the concealing layers of time and distance.

This issue is all about using technology to explore and expose those hidden worlds, whether they are in the ocean depths, in the far reaches of our galaxy, or swirling all around us, unseen. 

The moon that graces our cover is Europa, one of Jupiter’s many orbiting bodies. Europa is covered in ice with water below, and scientists have long wondered whether the planet could support life. Later this year, NASA will launch the Europa Clipper. The spacecraft is scheduled to reach Europa in 2030 and spend the next four years studying its environs. Stephen Ornes takes you to this mysterious Jovian moon. 

From 300 meters below the surface, Samantha Schuyler has the incredible story of a brute-force breakthrough that is allowing divers to go deeper than ever before possible. In an attempt to descend to these unprecedented depths in some of Florida’s deepest underwater caves, divers turned to a combustible gas that’s capable of sucking all the heat out of their bodies: hydrogen. 

In a story that also starts out under the water, Matthew Ponsford describes the effort to build an Internet of Animals. Scientists intend to monitor on the order of 100,000 animals—including rodents and birds—with GPS-equipped tags that can be tracked by a system of microsatellites in low Earth orbit. This network could reveal not only migration patterns and ways in which animal populations interact, but also new information about our planet itself. 

Back onterra firma, Meg Duff brings us a glimpse of a world that is all around us: Wi-Fi sensing. This technology, which is already in millions of homes, uses Wi-Fi waves to detect motion. For now, much of its usefulness is limited to detecting whether a space is occupied or not. But in the coming years it will quite literally be able to see through the walls of your home and monitor your movements inside. This means Wi-Fi sensing will be increasingly employed to monitor people’s health, their micro-­motions, and even their movements through crowded buildings. 

One other reason hidden worlds fascinate us so much is that they often, despite our best efforts, remain hidden. Dan Garisto brings us the story of how particle physicists are trying to push the field forward into the post-Higgs era. At stake are unanswered questions about the fundamental constituents of the universe.

And I hope you’ll find plenty of other stories to grab your attention—about everything from Antarctica to Wikipedia—hidden all throughout this issue. 

Thank you,

Mat Honan