world records have fallen four times -- but it's also great for people-watching. Many participants wear costumes. In 2016, more than 30 Guinness World Records were set for fastest times in various get-ups, including the fastest time in a film-character costume (2:39:09 by a man dressed as Elsa from "Frozen") and the fastest two-person three-legged race (3:07:57). A record 39,140 runners finished in 2016.

History: After dueling over the final miles in the 1981 debut race, American Dick Beardsley and Norway's Inge Simonsen -- who had never met before -- decided to cross the finish line hand-in-hand to tie for the victory.

Key feature: With elite fields and a flat course, London races often come down to the wire, so the finish line, with Buckingham Palace in the background, isgreat for spectators.

Scenic highlight: Runners pass the Cutty Sark, Big Ben, the Tower of London and the London Eye, cross the Tower Bridge and finish near Buckingham Palace.

Quotable: "It's fast, so for a Boston qualifier or for a personal best, that's certainly a place to go to run it," said Deena Kastor, who ran a U.S.-record 2:19:36 at London in 2006. "The weather's almost always perfect ... and paralleling the River Thames and running over the Tower Bridge and running towards Big Ben -- that big, beautiful clock luring you to the finish line -- comes with huge fanfare."

Marine Corps

Uniformed Marines provide support at the Marine Corps Marathon, which ends at the Iwo Jima Memorial in Arlington, Virginia. AP Photo/Kevin Wolf

It's been called "The People's Marathon," as it