On August 25th 1875, Briton Matthew Webb became the first verified person to swim across the English Channel without the use of artificial aids.
His first attempt to swim it was on August 12th and failed. On August 24th, he set off from Dover on his second attempt. 21 hours and 45 minutes later, on August 25th, he completed the 21 mile stretch and arrived at Calais in France, although currents, winds and the like meant that he actually swam a total of 39 miles.
The Straight of Dover is the narrowest portion of the Channel. The narrowness of the Channel amplifies the tidal range, and it is a more challenging body of water than might be expected. The fastest verified swimming time to date was done by Australian Trent Grimsby in 2012 who completed it in 6 hours 55 minutes.
Image: By Illustrated London News (Royal Museums Greenwich) [CC BY-SA 3.0 (creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons