It is amazing to run those numbers through the head and then try to imagine what that many years feels like. Civilizations have come and gone, seasons, storms - and yet the caves have remained largely the same.

Head spinning, I stepped out onto an overlook, where I could see the Rio Camuy very far below. It shot forward from one hole in the earth and then snaked along a narrow slot before entering the rock once more. How deep did it go? I wondered. How fast was that water raging? What would it be like to attempt that same journey in a white-water raft, bobbing and surging ever deeper into the cave before finally shooting back into the light?

Then, prompted by the guide, I continued making my way through the cave of wonders, taking pains to keep my hands on the sturdy metal rail and my feet on that wondrous concrete path.

To find the Rio Camuy Caverns, I took Highway 22 west from San Juan, and then headed south on Route 129, following the signs. That same road leads to the Arecibo Observatory. So it is that a small jungle highway with very few amenities connects the core of the earth and the conduit to deep space. The Arecibo Observatory must be seen to be believed, but imagine that an artificial lunar crater has been carved out of a jungle's limestone peak. The depression of the crater houses the largest radio telescope on earth, capable of peering far beyond our galaxy and into the billions of others galaxies that are said to lie somewhere out in the great beyond.

I'm not much of a science geek, so I don't have the talent to properly appreciate terms like light years and multiple aperture telescope, but I can comprehend the enormity of a concave structure 1,000 feet across, beaming signals into outer space. The radio telescope has been used to determine, for instance, the rotation rate of Mercury, the accurate location of spy satellites, the exact ­image of asteroids, and the existence of pulsar and extrasolar planets. And it's also made a pair of fine cameo appearances­ inI and in the James Bond film GoldenEye.