Short answers, pulled from the story.
Cairo, Illinois is located at the confluence of the Ohio and Mississippi rivers, at the southernmost tip of Illinois. It is the county seat of Alexander County and sits at the lowest elevation of any location in the state.
Cairo's population peaked at 15,203 in 1920 and fell to 1,733 by the 2020 census, an 89 percent loss over a century. The decline was driven by a series of railroad and highway bridges built between 1889 and 1978 that bypassed the city, ending the ferry and river trade industries that had sustained it. Decades of racial violence and a decade-long boycott of white-owned businesses in the 1960s and 1970s accelerated the economic collapse.
Cairo served as both a Union naval base and a major supply and training center during the Civil War. Admiral Andrew Hull Foote designated it the station for the Mississippi River Squadron on the 6th of September 1861. In January 1862, General Ulysses S. Grant occupied the city and ordered Fort Defiance built to control access to the Ohio and Mississippi rivers, using Cairo to launch and supply his campaigns south.
Racial tensions erupted in July 1967 after 19-year-old Robert Hunt was found dead in the Cairo police station, an event the Black community blamed on police. Rioting followed on the 17th of July 1967. By 1969, the Cairo United Front had organized a decade-long boycott of white-owned businesses and presented seven demands to the city including the appointment of a Black police chief. Illinois Governor Richard Ogilvie deployed National Guardsmen in the summer of 1969 to restore order.
The United States Custom House and Post Office in Cairo was designed by Treasury Supervising Architect Alfred B. Mullet and completed in 1872. At the height of Cairo's prosperity, its post office was the third busiest in the United States. The building is one of only seven surviving Victorian structures by Mullet in the entire country and is listed on the National Register of Historic Places; it has since been converted into a museum.
Cairo is completely enclosed by a system of levees and flood walls because of its low elevation between the Ohio and Mississippi rivers. A massive flood gate called the "Big Subway Gate," built in 1914 by Stupp Brothers of St. Louis, weighs 80 tons and seals the northern levee by closing over U.S. Highway 51. The current concrete flood wall is designed to protect the city from water levels up to 64 feet; the entire city was evacuated in 2011 when the Ohio River threatened to inundate it under 15 feet of water.