First, we needed a 4×4 of some sort, along with a driver willing to chance roads that are sometimes passable, sometimes not. The man we found struck us as the quietly skeptical sort, but after a few hundred rutted kilometers, any hesitations he’d been suppressing hardened into emphatic certainties. “The only people who drive on this road,” he told our photographer and me, via our translator, “are people who want to kill their cars.” Yet he gamely pushed ever deeper into Madagascar’s tropical north, until our mud road descended a hill and was swallowed by a wide river. It was the end of the line for the driver. He seemed relieved.

Somewhere on the other side of that water, dozens of farmers would soon converge upon a regional vanilla market in the village of Tanambao Betsivakiny. Growers would negotiate with buyers working on behalf of exporters and international flavoring companies, and together everyone would hash out a collective, per-kilogram price for the crop. Most buyers would pay cash on the spot, and the farmers would hand over several tons of green, freshly harvested vanilla beans.

Those humble beans, whose essence is associated with all that’s bland and unexciting, have somehow metamorphosed, butterfly-style, into the most flamboyantly mercurial commodity on the planet. In the past two decades, cured vanilla beans have been known to fetch almost $600 per kilogram one week, then $20 or so the next. Northeastern Madagascar is the world’s largest producer of natural vanilla, so every boom and every bust slams this region like a tropical storm. When prices peak, cash floods the villages. When prices fall, it drains away.

Madagascar was largely integrated into global trade centuries ago. The island is bigger than France, with cultural traditions that vary by region, unique biological treasures, and a developing tourism economy. The capital, Antananarivo, is full of laborers, lawyers, bureaucrats, bankers, artists, entrepreneurs, intellectuals—everything a 21st century city of 1.5 million needs. Yet Madagascar is also one of the poorest countries on the planet. You see and feel its disparities most sharply in its more remote pockets, including in the vanilla-growing region of the northeast. The extreme isolation of those communities, their dominance over the international supply, the dramatic changes they undergo during price swings—all of it has turned this part of the country into a semicontained observation lab that exposes both the genius and the insanity of globalized commerce. …