Campo de Perizes is not quite land and not quite ocean. It is a halophilous floodplain - vegetation adapted to salt - stretched between São Luís, Bacabeira, and Rosário, shaped by the Golfão Maranhense and the Baixada Maranhense region over millennia of marine transgression and regression. The Perizes River winds through it. The Itapecuru River empties into the sea nearby on the east; the Mearim flows in from the west. Some geographers consider the plain an extension of the larger Baixada Maranhense. Its distinguishing feature is the grasslands - thin, tough, salt-tolerant - that stretch to the horizon in every direction, broken only by stands of mangrove where the brackish water permits them.

The infrastructure crossing Campo de Perizes looks disproportionate to the land beneath it. Four distinct bridges span the channels and soft terrain: the entrance and exit bridges of the BR-135, the Benedito Leite Metallic Bridge belonging to the São Luís-Teresina Railway, the duplicated bridge of the Carajás Railway, and a separate metallic span carrying the Italuís water main, which pumps drinking water from the Itapecuru River into São Luís. The Carajás Railway is Brazil's iron-ore artery, running nearly 900 kilometers from the mines at Carajás in Pará to the port at Ponta da Madeira. Every bridge here carries something vital: water, iron, people. The plain beneath them is the wrong kind of ground to build on, and so everything has been built above.

The coast of Maranhão contains one of the largest contiguous mangrove systems in the world, and Campo de Perizes sits squarely within that ecology. Mangroves are one of the few plant communities that tolerate salt water directly at their roots, and their tangled aerial root systems shelter a rich community of crabs, mollusks, juvenile fish, and migratory birds. Where the rivers meet the tides, specialized vegetation thrives in water too brackish for almost anything else. The Perizes is not a spectacular river in the scenic sense - no waterfalls, no rapids - but it sustains a system of saltmarsh and mangrove that is as productive biologically as any rainforest. The grassland that gives the plain its name is simply the dry version of that same story.

The engineering challenge of crossing Campo de Perizes is hard to overstate. Soft, waterlogged, saline soil does not support highway loads. When the BR-135 duplication project began in 2012, the engineering team turned to stone columns - vertical shafts of compacted gravel installed every two meters along nearly 18 kilometers of the route. The depths varied from 5 to 18 meters depending on the local soil. The columns stabilized the ground enough to lay a modern divided highway on top. Water mains and railway tracks that had run alongside the old road were relocated. The technique had been used in shorter sections elsewhere in Brazil, but this was the first long application. Even so, within a few years of opening, portions of the road required reconstruction - a reminder that the plain still belongs to the water.

Campo de Perizes falls within the Upaon-Açu/Miritiba/Alto Preguiças Environmental Protection Area, a sprawling reserve created to preserve the region against the pressure of anthropization - a word environmental planners use when they do not want to say development. The pressure comes from everywhere: the highway and its traffic, the two railways, the water main, the electrical infrastructure, and the creeping urban expansion from São Luís, a metropolitan region of more than 1.5 million people now spilling toward the mainland. Discussions continue about creating additional conservation units to better protect the area. Meanwhile, the mangroves retreat in some places, the grasslands thin, and the wrong kind of ground keeps doing what the wrong kind of ground does, which is absorb whatever is placed on top of it.