On January 28, 1966, Ecuador declared Pululahua a national park - the first in the country and, by most accountings, the first in South America. It was a forward-looking act for the era, made because the place is geologically singular and biologically rich. On February 17, 1978, the protection was upgraded to geobotanical reserve, a rarer designation meant to acknowledge both the geology and the more than 2,000 plant species that flourish on the caldera walls. The Andes as a whole host some 2,900 endemic species; a disproportionate share of them live inside this single crater. Mosses, ferns, lichens, bromeliads, and orchids knit together the cloud-forest vegetation. Birds, mammals, and what the reserve's own literature calls "insects of exotic appearance" fill the trees.

After the old volcano emptied itself around 500 BC, the inner chamber could no longer support the summit, and the mountain fell inward - the classic recipe for a caldera. But the magma wasn't finished. In the five centuries that followed, three new lava domes pushed up through the floor: Pondoña in the center, El Chivo to the south ending in a sharp point, and Pan de Azúcar. Pondoña even acquired its own small crater on its eastern flank during a later eruption roughly 500 years after the main collapse. The highest point on the rim is Sincholagua hill, 3,356 meters on the northeast side. El Mirador, the observation terrace on the southwestern rim at 2,833 meters, offers the iconic view: a green bowl with three dome-islands, stitched across by farm terraces.

The oldest locals remember the lime kilns. Twelve of them survive inside the Pululahua area - tall round stone chimneys, three to five meters high, with internal diameters just over a meter. Limestone was pried from the caldera walls, loaded into the kiln with wood and coal, and burned for two or three days until the purified lime fell to the bottom. Mules carried it out of the crater by the hundreds. The white lime-wash that still covers the walls of colonial Quito's old town - the chalky brightness you see in every photograph of the UNESCO historic center - came from these kilns. Lime mixed with sand, water, and clay also bound the rocks of colonial walls. The elders say lime here was once as expensive as gold. The crater fed the Spanish city's bones.

Pululahua has been a highway since long before it was a reserve. The Yumbo, a commercial people of the cloud forests, likely traveled these paths a thousand years ago, linking the Pacific coast with the highland. An important Yumbo site at Tulipe, 29 kilometers southwest, remains connected to the crater by trails that cross the Santa Lucía and Maquicupuna reserves. To the northeast, the Caranqui had a pyramid at Alance; their path to Pululahua crosses the Río Guallabamba. The Inca used these same roads to extend their empire into Yumbo and Caranqui territory. The Spanish used them again to push north into Esmeraldas. Every civilization that moved through this part of the northern Andes left footprints on the same handful of trails. Most are now cut by modern roads, but some still run.

What makes Pululahua singular, even among the world's calderas, is that people are still growing food on its floor. Families raise maize, runner beans and fava beans, potatoes, vegetables, tree tomatoes, and alfalfa in the 2,500-year-old bowl. There are two hostels inside the caldera where travelers can sleep among the fields, and a small traffic of hikers climbs down the 1.4-kilometer trail from El Mirador - a 300-meter descent that takes about half an hour, though climbing back out takes longer. Horses carry those who don't want to walk. The Ministry of Environment maintains campgrounds at Moraspungo, by the entrance, and on the crater floor itself. The whole place is 17 kilometers from Quito, near the La Mitad del Mundo monument that marks the equator. Few journeys anywhere pack so much into so few kilometers: equatorial line, volcanic caldera, cloud forest, colonial lime kilns, and a quiet crater where someone is still plowing a field.