Under a sliver of moon, we, and seven other families and couples, followed Blue Caribe guide Alex (he wore a glow-stick) through a mangrove forest, where we slipped our kayaks into the lagoon. The short paddle into the bay, under cover of darkness, was eerily delightful. Constellations galore revealed themselves in the inky sky; the boys identified the Southern Cross, Orion, and the Milky Way. We've done lots of kayaking, but night paddling is a different experience altogether.

As we dipped our paddles into the water, we noticed something wondrous: the bay lit up with a shimmering streak with each stroke. Then, in the middle of the bay, we rolled out of our kayaks and into the bathtub-warm water. With each movement, we were covered with a million little diamonds of light. Swishing our arms and legs made dazzling vapor trails. The boys quickly launched into a Star Wars-ian battle of light sabers, courtesy of Mother Nature.

Were we doing harm to the environment by being here, we wondered? ''Not really,'' Alex said. ``We'll leave here with hundreds of them on us, but they reproduce at lightning-fast speeds, so no harm done.''

As he spoke, a shooting star fell from the sky, adding to the magic of the night.

The kayak tour ends at 9 p.m., after the last ferry back to Fajardo, so we'd booked a night in a nearby hotel, giving us time to enjoy a late-night walk along the beach and dinner in Esperanza.

CAVE-CRAWLING

Our heart-pounding, knee-scraping, muddy caving adventure began with a hot, sticky hike through the jungle, a zip line ride across a plunging gully, and a 250-foot rappel down a rocky cliff. And that was the easy part.

While most tourists opt for a relatively tame tram ride through Puerto Rico's Río Camuy Cave Park -- the third largest cave system in the world -- we followed island guide Rossano Boscarino to the mouth of Angeles Cave, a lesser-known section of the underground labyrinth. Headlamps, helmets, life jackets . . .

''We're ready,'' Boscarino said. ''Watch your footing; it's slippery. And watch where you place your hands. There are lots of guavas hiding in the crevices.'' Guavas? Those, we learned, were creepy spiders the size of dinner plates. There were plenty of them, along with thousands of bats and scurrying bugs.

For the better part of the day, we sloshed through knee-deep mud, crawled through narrow, wet tunnels and climbed slippery slopes. There were free jumps into dark pools of water and body rafting the underground, fast-moving river.

''Start swimming before you hit the water,'' Boscarino warned us. ``You don't want the river to take you away.''

Everywhere were strange sea creature fossils, large calcite water drippings, and fascinating formations. Lunch was served in a chamber the size of a cathedral, decorated with giant flowstones, hanging stalagmites and glistening stalactites.

The underground journey ended at the top of a two-story-high mudslide. We rode it to the bottom, popping out, mud-caked and tired, into a bright, sun dappled forest.

''Sick!'' the boys exclaimed, high-fiving. We couldn't have agreed more.