After years at sea, a salmon returns to the exact river it was born in

Salmon hatch in freshwater rivers and streams, then swim downstream to the ocean, where they spend anywhere from one to several years, sometimes covering thousands of kilometers of open water.

When it's time to spawn, a salmon returns — and not to just any river, but with astonishing precision to the very river, often the very stretch of stream, where it hatched.

The mechanism relies mainly on smell: as a juvenile, a salmon memorizes the unique chemical makeup of its home stream's water, and years later, upon reaching the river mouth, it recognizes that "scent" and follows it upstream all the way to the source.

Along the way, salmon leap up waterfalls against the current, despite exhaustion and attacks from bears and eagles — all of that effort serves one purpose: getting back to exactly where their life began.