Short answers, pulled from the story.
The English Channel is an arm of the Atlantic Ocean that separates southern England from northern France. It is about 560 km long, varies in width from 240 km at its widest to 34 km at its narrowest in the Strait of Dover, and covers an area of some 75,000 km2.
The Channel formed during the Pleistocene period through at least two catastrophic glacial lake outburst floods that breached the Weald-Artois anticline ridge joining Britain to continental Europe. The first flood, around 450,000 years ago, released an estimated one million cubic metres of water per second. A second flood around 180,000 years ago carved the Lobourg Channel through the Strait of Dover.
Captain Matthew Webb made the first observed and unassisted swim across the English Channel, swimming from England to France on the 24th-the 25th of August 1875 in 21 hours 45 minutes. The fastest verified solo crossing is held by Australian Trent Grimsey, who completed the swim on the 8th of September 2012 in 6 hours 55 minutes.
Louis Blériot of France piloted the first aeroplane to cross the English Channel on the 25th of July 1909, flying from Calais to Dover. The first balloon crossing had been made earlier, in 1785, by Jean Pierre François Blanchard and John Jeffries.
The Channel Tunnel opened in 1994, connecting the United Kingdom and France by rail. Eurostar trains run between London, Paris, and Brussels, while Eurotunnel Shuttle trains carry cars, coaches, and lorries between Folkestone and Calais.
The Channel was central to several major operations in the Second World War. In May-June 1940, it enabled the evacuation of 338,000 Allied troops from Dunkirk in Operation Dynamo and a further 192,000 from other ports in Operation Aerial. In June 1944 it was the crossing point for Operation Overlord, the Normandy Landings. The Channel Islands, which lie in the Channel off the French coast, were the only part of the British Commonwealth occupied by Germany during the war, from 1940 to 1945.