It is the third oldest in the world.
On December 14, 1894, during the construction process, 13 workmen who were building a tunnel under the Clyde were trapped by a fire.
Imprisoned for 12 hours before being rescued, they only survived by lying beneath the thick smoke and passing a small airpipe from mouth to mouth.
After five years' work the system was complete, and at 5am on December 14, 1896, the first train left Govan.
No fewer than 25,000 people took advantage of the first day reduced fare of one penny, swamping the stations.
There were several minor accidents, including disorder that required police intervention, the St Enoch stationmaster being pushed on to the lines by the crowd, and a collision between two trains in a tunnel between Buchanan Street and St Enoch.
The next day the Subway was closed, reopening five weeks later once the initial troubles had been fixed.
The trains were not electrified but were hauled by a moving cable that was kept at a continuous speed of 13mph by a coal-fired power station on Scotland Street.
The cable, however, sometimes struggled with the gradient between Govan and Partick.
The cable was removed in 1935, and was replaced by electrified tracks and new trains. The subway was also renamed the Glasgow Underground.
The underground closed in 1977 for refurbishment, and during the process a seam of top-quality coal was discovered beneath St George's Cross station.
Over five months, workmen used their lunch-breaks to load 16 tons' worth of coal into sacks. What remains of the seam now lies beneath the station's foundations.