The world we think of as the earth is a very small fragment of its true extent. Vegetation and oceans sit at the top, a thin layer over a deep bed of solid rock that itself becomes molten and penetrates thousands of kilometers further down. Rarely do we get to experience this subterranean world, though its mysteries have intrigued us for centuries. Take, for example, Athanasius Kircher’s Mundus subterraneus, a 17th century treatise on volcanism, the interior of the earth, and speleology (a beautiful digitzed version can be found here). Or consider the ancient tunnel system at the Phlegraean Fields near modern Naples, home to the Cumaean sibyl and an underground boiling river meant to simulate the entrance to the underword. What really occurs in these underground worlds?

I recently had the opportunity to explore a large cave, and thought long about how the biology I study reflects on only a small part of the larger earth system. This cave was in the center of Puerto Rico, a large and winding system cut out over many years into the limestone karst. We descended into darkness, clambering through the roots of a large fig tree, and found ourselves in a large cavern.

A few hundred meters’ walk brought us into complete darkness through a windy series of passages. But in this darkness, a chance flick of my headlamp revealed something thoroughly unexpected, that brought my mind back to life at the surface.

Somehow, deep in this darkness, a long Ficus root had extended itself, hundreds of meters from the cave entrance. And this root had put out a new stem, and on that stem, a leaf! Leaves are for photosynthesis, and in the absence of light, require large amounts of carbon for respiration and maintenance. This was the least useful leaf I’ve ever seen, and I have no explanation for why this plant would have been driven to grow a new stem here. Plants are quite capable of sensing light, and will usually drop leaves that no longer serve a useful photosynthetic purpose.

The cave passages continued to wind and turn, and eventually we found ourselves in the light once more – an exit to the cave, not one that was easily taken. Our passageway opened up into a wide window on the edge of a precipitous cliff, with a view of all the valley beneath us. Here my worlds of darkness and light met, and I was happy, sitting for just a few hours with the breeze blowing in, glad to contemplate the mystery of my subterranean Ficus root.