This piece depicts a boulder of serpentinite on Marshall Beach of the Presidio of San Fransisco, half a mile south of the Golden Gate Bridge (37.801869, -122.479966). The rock is an altered slice of the mantle beneath the ocean crust, exhumed by tectonic processes associated with subduction in on the western coast of the United States. The green color reflects the ultramafic (iron-rich) protolith for this metamorphic rock. But in its journey for the surface, this rock underwent great change. At both the mid ocean ridge, where this rock was made, at the subduction zone, where it goes to die, hydrothermal fluids worked their magic on this rock. These fluids altered the rock’s chemistry and caused the development of carbonate minerals. These minerals grew from carbon in ocean water, sequestering CO2 into rocks, where it will rest for millions of years. Scientists and entrepreneurs want to accelerate and apply the chemical reaction that made this rock to draw down CO2 from the atmosphere.