In the Matrix franchise, 01 (also referred to as Zero-One and the Machine City) is a city-state established by The Machines. During the Second Renaissance, following the trial of B1-66-ER, the nations of the world feared the possibility of the intelligent machines they had created revolting against humanity, and so called for the eradication of all artificial intelligence. In order to escape the violence of their human makers' oppression, the Machines founded their own independent nation of 01 and attempted to live peacefully alongside mankind. The peace was short-lived, unfortunately, since the Machine nation's prosperity was damaging the global economy, leading to the human nations of the world calling for a complete blockade of trade with the Machines. The Machines attempted to settle this dispute diplomatically and made an appeal to the United Nations, but the UN unanimously voted against 01's entry and even went so far as to assault the Machine ambassadors.
Shortly after the incident at the UN, the Machine War broke out and humanity was swiftly dominated by the Machines. Though 01 suffered damage during the war, the machine city-state was quickly repaired and enhanced. In the decades since the war began, 01 has grown into a nightmarish cityscape blanketed in darkness, a perpetual fog drifting amidst its many gleaming black spires. Machines of all shapes and sizes dwell within the city, most of which humans have no understanding of their function or purpose. Throughout the war between Zion and the Machines, no human has ever entered 01 and come out alive. Even Neo's one visit to the Machine City was no exception.
In The Matrix Revolutions, Neo and Trinity travelled to 01 in order to meet directly with the Machine leaders and reach an agreement to put an end to the war. Their ship barely made it past the city's defensive border and Trinity died when the ship crashed. Neo continued onward into the heart of 01 and met with the Deus Ex Machina, informing it that the renegade Agent program called Smith had evolved into a virus and would soon destroy The Matrix and even threaten the real world. Neo offered himself to the Machines, promising to stop Smith in exchange for peace between humans and Machines.