As an amateur economist, it's always nice to be reminded that people respond to incentives. Many houses that we see in Vietnam are quite beautiful... on one, very tall and skinny side. Apparently, this is what you get when you tax houses based on their width.

We decide to take a three-day tour of Halong Bay. Our boat is much nicer than the hostels we're used to staying at.

As we float out into the bay, we can't see too much because of the mist. After a while, limestone karsts gradually take shape and loom over us. Soon, we're surrounded by eerie monoliths on all sides.

Our boat pulls into a group of islands where a large crowd of others has already anchored. The islands would probably look gorgeous if they weren't overrun. April gets on a small boat for a trip to the Cave of Surprises. Through an unpleasant sequence of events involving our poor knowledge of Vietnamese, Paul gets left behind on the main boat. That's OK though - Paul gets two free beers in the end, and anyway, the natural beauty of the cave has long been spoiled by crowds, handrails, and tacky lighting... Or at least that's what April tells Paul afterward to make him feel better.

When we wake up, the fog has receded a bit, and we enjoy clearer views of the karsts.

Our boat drops us off at Cat Ba Island, one of the largest islands in the bay, where we find ourselves at the entrance to another cave. At first, this one is... ok. The cave is pretty, but there's artificial lighting and stairs have been carved into the floor. We're not sure how we feel about the large Buddhist altar, built to honor the earlier people whose remains were found in the cave. When we reach what looks like the end, our guide squats down and shines light in a corner, where he claims there's a narrow passage, big enough to slide through on your belly. Apparently, the rooms on the other side are the most beautiful and untouched in the cave. On occasion, our guide says, "the very brave" can go through and visit.

At first, it sounds like he's speaking about some professional spelunking team, and we assume that our group will have to turn around at this point and head to lunch. After some prodding, though, we realize that our guide really is inviting us to try the passage for ourselves (Or maybe he feels bad that Paul missed yesterday's cave).

April is the first to squeeze through (of course), exclaiming that she feels the beat of a bat's wings by her head. Paul follows with a little more trouble, afraid that his shorts might remain clamped between the rocks as he slips through. But turning back would be embarrassing as well, so he makes it through with a heave, and the shorts survive. One more group member joins us on the other side.

The deeper chambers are well worth a frightening squeeze and then some. The walls are draped with sparking curtains, hollow stalagmites rise from the floor, and delicate straws hang above our heads. Water drops glisten everywhere we shine our flashlights. We stay here for a few minutes, enjoying the company of bats and the feeling of being explorers.

After lunch, we're given the rest of the day as free-time, and we're happy to be independent again. Looking at the map, we realize that "Monkey Island" is only a mile or so offshore, so we find a kayak rental and set out across the water by ourselves. A surprising number of people live right on the water in floating houses, and an unfortunate side effect is a lot of trash in the water. We turn around a karst and out of the harbor, though, and we're surrounded by clear water and beautiful scenery (and slightly intimidating waves). A few hundred more strokes, and we pull up on the sandy beach of Monkey Island.

The guide book says that a troop of monkeys lives here, and is famous for biting tourists (really gotta get that rabies vaccine), so we set off to find them. A rocky path ascends from the beach up into the trees, with an occasional rope to help you up. As we climb, the limestone steadily becomes sharper. When we leave the path to try to summit the peak of the island, we're virtually walking on knife edges. This would be worrisome for any hiker, but especially for us, since April brought only her sandals to walk in, and these are in visibly worse shape than when we started. Despite the hazards, we gingerly pick our way up to the top. The monkeys are nowhere to be found, but the view is spectacular.

Before we leave the island, we perform our obligatory vacation gymnastics.

We sail back out through the karsts to the mainland, and feel sad to be leaving them behind. Today will mostly be a travel day: A few hours' drive to Hanoi, a brief stop for tropical fruit, and jump on a sleeper train heading south to even warmer places. But first, one last look at the islands.