John Day (c. 1740 – died 22 June 1774) is the first recorded death in an accident with a submarine. Day was an English carpenter and wheelwright. With the financial support of Christopher Blake, an English gambler, Day built a wooden "diving chamber" without an engine. He attached his invention to the deck of a 50-ton sloop named the Maria, which Blake had purchased for £340. The sloop's hold contained 10 tons of ballast, and two 10-ton weights were attached beneath the keel which could be released from inside the diving chamber. An additional 20 tons of ballast would be loaded on the Maria after Day had been locked inside the diving chamber.
On 22 June 1774, the Maria was towed to a location north of Drake's Island (St. Nicholas' Island) in Plymouth Sound, off Plymouth, England. Day took a candle, water and biscuits on board. The boat was equipped with a hammock for the passenger. After the boat was locked, the weights were loaded and the boat sank forever into the depths. Day had the calculation of the trim completely wrong. It has been speculated that Day may have died from asphyxiation, hypothermia or catastrophic structural failure of the Maria and/or the diving chamber due to water pressure. This incident was the first recorded fatal accident involving a submarine.
- 22 June thebookofdays.com (Retrieved on 7 June 2011)
- Chambers, Robert (30 October 1888). "The Book of Days: A Miscellany of Popular Antiquities in Connection with the Calendar, Including Anecdote, Biography & History, Curiosities of Literature, and Oddities of Human Life and Character". W. & R. Chambers. Retrieved 30 October 2017 – via Google Books.
- Churchill, Dennis (2011). "The First Submariner Casualty" (PDF). In Depth (32): 5–6. Archived from the original (PDF) on 23 July 2011. Retrieved 7 June 2011.
- Tall, Jeffrey (2002). Submarines & Deep-Sea Vehicles. Thunder Bay Press. pp. 13–14. ISBN 978-1-57145-778-3.
- Ecott, Tim (2001). Neutral Buoyancy: Adventures in a Liquid World. New York: Atlantic Monthly Press. pp. 237–240. ISBN 0-87113-794-1. LCCN 2001018840.
- McCartney, Innes (2003). Lost Patrols: Submarine Wrecks of the English Channel. Penzance: Periscope Publishing Ltd. pp. 48–49. ISBN 1-904381-04-9.
- This date comes from The Book of Days. According to Churchill and McCartney, Day's fatal dive occurred on 20 June 1774.