BBC ScotlandIt follows a line that's as old as the hills, and now Scotland's newest long-distance footpath has been marked out for hikers.
The 745-mile (1,200km) route follows the watershed, the line dividing river systems that flow west to the Atlantic from those that end up in the North Sea.
"Imagine you are a raindrop", Peter Wright, the new route's author, told me.
Frankly, that wasn't difficult.
On the day I met him in the Campsie Fells the cloud was low. The fog was high. And the drizzle was pretty constant.
"When you fall to ground you either have to go west, to the Atlantic Ocean. Or east, and end up in the North Sea."
Look an interesting route.