The Pongo de Manseriche is a geographic threshold. On the upstream side, the Maranon still bears the character of a mountain river - cold, fast, broken. Below the gorge, everything changes. The river slows and broadens. The banks flatten. The jungle takes over. Borja sits on these lowland banks in what is now the Datem del Maranon Province of Peru's Loreto Region. In 1619, Diego Vaca de Vega, a settler from Loja in what is now Ecuador, founded the town as one of the first Spanish settlements in the lowland Amazon Basin. He named it Borja in honor of Francisco de Borja y Aragon, then Viceroy of Peru. Francisco was descended from the infamous Spanish Borgia family, a great-grandson of the Borja saint Francis of Borja who had reformed the Jesuit Order in Europe. The choice of name proved prophetic. Jesuits would shape the town's entire history.

The land Borja occupied had long belonged to the Maina people, one of many indigenous groups in the Amazon headwaters. At the time of Spanish arrival, Maina numbered three or four thousand in the immediate region. The colonial government granted encomiendas to twenty-one Spaniards and mestizos - effectively the legal right to compel labor from indigenous people - and the Maina were forced to work on Spanish farms. Many died of diseases against which they had no immunity. Others died from the brutality of forced labor. In 1635, the Maina rose in revolt. The rebellion was severely repressed. Three years later, in 1638, Jesuit missionaries arrived to establish what they called the Mainas Missions.

When the Jesuits arrived, Borja's population consisted of about 2,800 Maina and other indigenous peoples, plus 200 Spaniards including a small military garrison. The Jesuits operated from Borja as an administrative center for an enormous mission territory - a region they called Mainas, as large as Spain itself. Their method was straightforward and grim: missionaries, soldiers, and Christianized indigenous people mounted expeditions into the surrounding jungle to capture members of uncontacted or unsubmitted groups and bring them to the settlement. The settlements they created, the reducciones, were meant to concentrate dispersed peoples into permanent Christian villages. Most of these missions were ephemeral. They collapsed when resistance mounted, when disease swept through, when rivers shifted or crops failed. Of the hundreds of missions established across the region, most no longer exist.

Despite the expeditions, despite the forced concentration of populations, Borja continued to lose people. By 1661, the population had fallen to about 1,000. By 1776, it was 263 indigenous people and 152 Spanish and mestizos - fewer than six hundred souls total in what had been one of Spain's administrative centers for the upper Amazon. Disease did much of the work. Escape and displacement did the rest. The town's geographic isolation made everything harder. The nearest Spanish highland settlement, Jaen, was theoretically four days away by road - but only at the most favorable times of year. The return journey, floating downriver, took two and a half days through the treacherous Pongo. When the water was high, the river was simply not navigable, and Borja was cut off entirely. The town produced small amounts of tobacco and achiote - the red dye plant used by indigenous peoples throughout the Amazon - for trade with Jaen.

The Jesuits were expelled from the Spanish Empire in 1767, ending their missions across the Americas. Borja survived, barely. Today it is a hamlet of 329 people on the banks of the Maranon, still accessible only by river. The Pongo de Manseriche still rushes past just upstream. The Maina as a distinct people have largely been absorbed into broader categories - many of their descendants now identify as Jivaroan or Awajun. The name Mainas survives on maps of the region. Standing on the riverbank at Borja, watching the current roll east toward the distant Atlantic, it is possible to feel the weight of what was lost here: not a city that rose and fell, but a concentration point for a catastrophe that fell on peoples whose names many of us have never learned to pronounce.