New islands are supposed to take ages—carved out by slow tectonic shifts or patiently built by coral over centuries. But in the last 100 years, Earth has been caught in the act of making land in real time. Some of these islands blasted out of the sea in a single day, born from underwater volcanoes and ash clouds so thick they turned noon into twilight.

Others appeared more quietly—river mud stacking up year after year until a “sandbar” became a coastline, then a real island with plants, birds, and eventually people. And a few weren’t made by nature at all, but by humans—dredged, dumped, and engineered into brand-new territory that didn’t exist on maps a generation ago. In today’s video, we’re counting down 15 islands formed in the past 100 years—where they came from, how fast they grew, and which ones might vanish as quickly as they arrived.