How far will you walk over rather than down? How much farther will you walk over rather climb up?

As an architect who designed large, multi-story buildings, I learned early on to locate physical spaces that have important connections next to each other rather than above or below. Why: because people prefer to walk horizontally rather than take stairs or even elevators. This preference for horizontal travel is irrational. Most everyone will opt for 100, 200, even 500 feet horizontally over 15 feet vertically. I accepted this quirk of human nature, and laid out spaces accordingly. But recently came to appreciate it in a deeper way.

I have a cursory curiosity about most everything with an engineering temperament to match. Math and science are tools I use to affect the world, hopefully in a positive way. But I am more adept at connection and breadth than rigorous depth. My friend Bob, on the other hand, is pure scientist, a geologist by training, curious about the natural world without pesky regard for how we might manipulate it. Recently, Bob stayed with us. During back-to-back dinner conversations, unrelated to architectural design, he gave me unexpected perspective on why we humans have such a strong preference for horizontal space.

On a pleasant Saturday, Bob and I rode our bikes out the Minuteman Trail to Concord’s Old North Bridge. Which, having been replaced five times since ‘The Shot Heard ‘Round the World, is actually not very old, but I digress. The sixteen-mile ride is a lovely trek on a spring day. The trail, on an old railroad grade, has scant vertical rise, though on a bicycle you notice even one percent grade.

That evening, Bob was in geologist mode. He talked at length about the earth’s crust, and the Magma that we more of less float upon. The distance from the Mariana Trench to Mount Everest, the deepest and tallest extents of our crust, are a mere 12.3 vertical miles. We estimate the crust itself is up to thirty miles thick below continents, as little as three miles thick under the oceans, which means an average thickness of about sixteen miles, or the distance from Cambridge to Old North Bridge. Yet we have not actually gotten to the bottom of the earths crust; the deepest hole ever dug, the Kola Superdeep Borehole in Russia, goes only 7.5 miles below the surface.

The next day was rainy; Bob spent most of his time reading Jeffrey Kluger’s Apollo 8: The Thrilling Story of the First Mission to the Moon. Therefore, most of our dinner talk centered around space. Sixteen miles above the earth? That’s about twice the distance beyond the limits of our atmosphere.

Taken together, I realized what a thin slice of space we humans occupy. We occupy a 25,000 mile circumferential sphere that we can wander upon with relative ease. But we really don’t know much about eight miles below us, and we are dead eight miles up. Suddenly our predilection for moving horizontally does not seem irrational; it seems downright instinctual.