In between two of the islands of Indonesia, there's an ancient line that is both real, and not real.
You can't see it, but it's there all the same.
If you stood on the coast of Bali and looked east to the shores of Lombok, you'd be staring right at the line's narrowest point: a 32 km stretch of water that seems pretty unassuming.
This invisible barrier weaves its way through the entire Malay Archipelago, the largest collection of islands on the planet.
See, on the western side, the animal life is characteristic of Asia, featuring rhinos, elephants, tigers, and woodpeckers, to name a few.
But cross the line, and things suddenly change.
You won't find those same species on the eastern side.
Instead, the islands have a totally different cast of ecological characters, including marsupials, Komodo dragons, cockatoos, and honeyeaters.
This is what scientists call a biogeographic boundary, the meeting point of two regions of biodiversity that are highly distinct.
And this particular line, called the Wallace Line, is perhaps the sharpest and most iconic of all.