Right now, someone in Tokyo is watching a cat video uploaded in New York. Someone in London is video-calling their friend in Sydney. All those hellos and silly memes and movie streams — they're zipping across the ocean floor in cables thicker than a garden hose, carrying light.

The cables aren't full of water or electricity — they're full of glass. Thin strands of glass, each one narrower than a human hair, bundled together inside rubber and steel. And through that glass? Pulses of laser light, flashing on and off millions of times per second.

Every photo, every text, every "how do I fix a leaky faucet" search gets translated into a pattern of light blinks. On-off-on-on-off. That's how computers talk: everything becomes ones and zeros, and ones and zeros become light flashing through glass. Your lunch photo? A billion tiny blinks, fired into the cable.

But light has a problem. It gets tired. After about fifty miles of zooming through glass, the pulses start to blur and fade, like a whisper getting quieter the farther it travels. So every fifty miles or so, the cable has a booster station — a machine the size of a minivan, sitting on the seafloor, that catches the faint light and fires it out fresh and strong again.

How did the cable get down there in the first place? A ship with a giant spool on its deck, like the world's biggest fishing reel. The ship creeps across the ocean at walking speed, slowly feeding the cable overboard. It sinks through two miles of water, settles onto mud and rock and the bones of ancient coral, and stays there.

The cables don't go in straight lines. They zigzag around underwater mountains. They dodge earthquake zones where the seafloor cracks and shifts. Near the coast, where fishing nets and ship anchors could snag them, the cables get buried under the sand by a plow-robot dragged along the bottom.

Sometimes a cable breaks — a ship drags an anchor through it, or an undersea landslide buries it. When that happens, a repair ship sails out, drops a robot submarine with cameras and mechanical claws, finds the broken ends in the muck, hauls them up, splices them back together with a glass-welding tool, and lowers the fixed cable back down. The internet hiccups for a few hours, then keeps flowing.

Today, there are more than four hundred cables crisscrossing the ocean floor, carrying ninety-nine percent of the world's internet traffic. Every email to Grandma, every video of a dog learning to skateboard, every voice call across the sea — they all ride beams of light through hair-thin glass, shot through the dark at the bottom of the world. All because we figured out how to make glass so pure that light can travel through fifty miles of it and still shine.