Stone Avenue Underpass, Railway bridge in downtown Tucson, United States.

Stone Avenue Underpass is a concrete structure completed in January 1936 that carries two lanes in each direction of Stone Avenue beneath the Union Pacific railroad tracks in downtown Tucson. It runs between 6th Street and E. Toole Avenue/W. Franklin Street.

The underpass was built in 1936 as a concrete structure and gained listing on the National Register of Historic Places in 1988. This recognition came decades later, acknowledging its role as a Depression-era infrastructure project.

The nickname Lake Elmira was coined in summer 1937 by Arizona Daily Star reporter Howard Owetly, after 13-year-old Elmira Doakes, daughter of Joseph Doakes of Tucson. A 1937 newspaper account credits her as the first person to swim across the rainwater that pooled in the underpass, from the Toole Avenue Landing to the Northern Shore.

Stone Avenue carries two lanes in each direction between 6th Street and Toole Avenue/Franklin Street, passing under the Union Pacific tracks. Drainage is poor, so the passage floods fast in heavy downpours; check flood warnings and plan an alternate route during storms.

In the 1980s, two history enthusiasts placed an unauthorized zinc plaque on a pillar to preserve the Lake Elmira story. This unofficial marker shows how residents took it upon themselves to document a local legend.