For most of the twentieth century, ships were designed around the Panama Canal's locks. The maximum vessel size that could squeeze through -- 965 feet long, 106 feet wide -- became a global standard known as Panamax. Naval architects, shipbuilders, and port operators all oriented their work around these dimensions. But the shipping industry's appetite for scale outpaced the canal's constraints. Containerized cargo had overtaken dry bulk as the canal's top revenue generator, and the containers kept getting stacked higher on ever-larger hulls. The Panama Canal Authority estimated the waterway would reach its maximum sustainable capacity between 2009 and 2012. Every canal-widening study since the 1930s had reached the same conclusion: the best solution was a third set of locks large enough to handle the next generation of ships.

On April 24, 2006, Panamanian President Martin Torrijos formally proposed the expansion project. It was not a decision the government could make alone. On October 22, 2006, Panamanians went to the polls in a national referendum, and roughly 77 percent voted in favor. The project's scope was enormous: two entirely new lock complexes -- one on the Atlantic side, one on the Pacific -- connected by new, wider access channels. The existing navigational channels would be widened and deepened, and the maximum operating water level of Gatun Lake would be raised. The price tag was initially estimated at $5.25 billion, making it one of the largest infrastructure projects in the Western Hemisphere. The new locks would accommodate vessels called New Panamax -- roughly one and a half times larger than the old Panamax standard, capable of carrying more than twice the cargo.

Construction began in September 2007, led by a consortium called Grupo Unidos por el Canal, which included the Spanish firm Sacyr, the Italian firm Impregilo, the Belgian firm Jan De Nul, and the Panamanian firm Constructora Urbana. Each new lock chamber used water-saving basins designed to recycle 60 percent of the water from each lockage, addressing concerns about the strain on Gatun Lake's freshwater supply. The Atlantic locks, called Agua Clara, and the Pacific locks, called Cocoli, each feature three chambers capable of lifting New Panamax vessels 85 feet above sea level -- the same elevation as the original canal, but through much wider gates. The project ran into delays and cost overruns, with disputes between the consortium and the Panama Canal Authority adding years to the timeline.

The expanded canal began commercial operation on June 26, 2016 -- nearly two years behind the original schedule and significantly over budget. The first New Panamax vessel to transit the new locks was the Chinese-owned container ship Cosco Shipping Panama, carrying 9,472 twenty-foot equivalent units of cargo. The old Panamax locks, still operational beside the new ones, suddenly looked quaint by comparison. The expansion effectively doubled the canal's cargo capacity and opened it to liquefied natural gas tankers, larger bulk carriers, and the mega-container ships that now dominate global trade routes. Panama's toll revenues surged, vindicating the investment even as critics continued to debate the environmental and social costs.

The expansion's most persistent challenge was not concrete or steel but water. Each lockage through the new chambers consumes enormous volumes from Gatun Lake, and the water-saving basins, while helpful, do not eliminate the fundamental dependence on rainfall in the Chagres River watershed. Drought years, exacerbated by shifting climate patterns, have forced the canal authority to impose draft restrictions on transiting vessels, limiting how deeply loaded they can sit in the water. The tension between growing shipping demand and finite freshwater supply defines the canal's future. From the air, the twin lock systems are unmistakable -- the original 1914 chambers narrow and paired, the 2016 additions broad and flanked by their recycling basins. Two engineering eras sit side by side, both dependent on the same tropical rain falling into the same river valley.