Bean and I have been exploring Andy Goldsworthy’s art installations in the Presidio. Yesterday, we finally made it to Spire, a carved 90 foot peak jutting out of the surrounding grassland.

Thanks to its composition, though, Spire ended up being even cooler than it looked. To create it, Mr. Goldsworthy bundled 38 Monterey cypress tree trunks, amassing them into the 90 foot spire. From a distance, all the pieces merge to appear as a massive trunk shooting into the sky. Up close, you can see the interwoven trunks.

The spaces between the tree trunks are the key to the trick.

Inspecting the installation yesterday with a friend, we suddenly heard her toddler screaming for Mommy. Her voice was small and distant. She’d somehow managed to run 20 yards away, and was probably lost in the woods. We both went into freaked-out-mom-mode, and raced into the surrounding forest. Then we heard giggling behind us.

It turns out if one person leans into the installation and screams, a person listening on the other side will hear their voice thrown. It’s a fascinating deception, likely caused by the sound waves refracting around the tree trunks before they emerge into the other side. The sound movement mimicks the way sound moves through a forest, which is why we thought the toddler had run 20 yards away, into the (actual) forest.

Give it a try the next time you’re on a hike in the Presidio. It is downright eerie!