What happened here?

Andy Goldsworthy created a sculpture on a wall adjacent to the Officers' Club in The Presidio. It's called "Earth Wall," and indeed the wall you see in the picture is made of rammed earth.

Goldsworthy began by attaching curved eucalyptus branches to an existing concrete garden wall to create the ball in the middle. Contractors then built a temporary plywood wall parallel to the concrete wall and filled the space in between with a mixture of earth and cement. 

When the plywood wall was taken away, the eucalyptus ball was completely submerged in the rammed earth wall. Goldsworthy then excavated the ball with a hammer and chisel until it was partially revealed. 

He chose to make this piece of sculpture an excavation because the Officers Club is the site of an ongoing archaeological dig. Inside the building, visitors can now view an adobe wall that was part of the Spanish presidio (fortress) built in 1776. Next to the building is a dig that explores other parts of that 250-year-old building. 

All of Goldsworthy's work is site-specific. It reflects the site on which it is created and is made of materials from the site.