I take a lunchtime walk to 28th and Ortega. I start at the top of the reservoir. Most days, the fog is in. But on clear days, I see the Farallon Islands at the edge of the horizon.

The Farallones are mysterious. There are no people on them. Some days, the cool ocean air distorts their shape. They look like towering pillars. They are a mirage any day you see them.

But the strangest thing? They disappear as you walk toward them. Move west down Ortega Street and they sink lower and lower until they’re gone. It’s the strangest thing.

Why do they sink away?

The Earth is curved. The Farallones are 28 miles away, far enough to be hidden by the planet’s curvature. We only see them because the Sunset District slopes upward. The reservoir is 300 feet above sea level.

But walk down to the shore and you only see the very tip of the tallest Farallone.

A reminder that even here, in San Francisco, the world isn’t flat.