Japan Airlines Flight 123 was a scheduled domestic Japan Airlines passenger flight from Tokyo's Haneda Airport to Osaka International Airport, Japan. On Monday, August 12, 1985, a Boeing 747SR operating this route suffered an explosive decompression 12 minutes into the flight and, 32 minutes later, crashed into two ridges of Mount Takamagahara in Ueno, Gunma Prefecture, 100 kilometres (62 miles) from Tokyo. The crash site was on Osutaka Ridge (御巣鷹の尾根 Osutaka-no-One?), near Mount Osutaka. All 15 crew members and 505 of the 509 passengers on board died. The death toll was exacerbated by delays in the rescue operation.
The explosive decompression was caused by a faulty repair performed after a tailstrike incident during a landing seven years earlier. A doubler plate on the rear bulkhead of the plane was improperly repaired, compromising the plane's airworthiness. Cabin pressurization continued to expand and contract the improperly repaired bulkhead until the day of the accident, when the faulty repair finally failed, causing the explosive decompression that ripped off a large portion of the tail and caused the loss of hydraulic controls to the entire plane.