Fans of aviation or industrial design mourned the retirement of the Concorde in 2003. With it ended for the foreseeable future the brief era of supersonic passenger flight. Its retirement – which had nothing to do with the spectacular, tragic crash of Air France Flight 4590 in 2000 and everything to do with the plane's astronomical per-passenger cost to operate – was the first nod to economic reality in its history. The Concorde never made economic sense; it was finished as a matter of political and nationalistic pride in France and the UK. A textbook example of the sunk costs fallacy – which is now sometimes called the Concorde Fallacy in its honor – its existence was more a matter of 'can' than 'should'.
Flying at twice the speed of sound at 60,000 feet presented some unique challenges aside from the impractical economics. At the beginning of every commercial flight, FAA rules require flight attendants to give you a safety demonstration that you ignore completely; part of it is the infamous oxygen mask with its plastic bag that may or may not inflate (but don't worry, because either way oxygen is still flowing). The supplemental oxygen is there in case of a loss of cabin pressure. A normal jet cruising at 25-30,000 feet can make use of a passive system like this in case of emergency while the pilots descend to a safer altitude. There is very little oxygen or atmospheric pressure at 30,000 feet, so the goal is to give everyone enough oxygen to avoid hypoxia while quickly reaching a more breathable altitude.
But at 60,000 feet in the Concorde the engineers discovered that the mask-and-baggie system doesn't work. The air is too thin and the pressure too weak at that altitude. In the event of a loss of cabin pressure, the air at that altitude would be so thin that you couldn't even inhale well enough to stay conscious. The only way to ensure adequate oxygen would be to uses a positive pressure device like a CPAP (similar to what people with sleep apnea use). Installing one for each of 100 passengers in the Concorde was totally impractical in terms of cost and weight.
Instead, they installed a positive-pressure system in the cockpit with fighter pilot-style oxygen masks to ensure that the flight crew would be OK. Then they calculated what is known as the "time of useful consciousness" (TOC) at 60,000 feet – how long it will take the average human to pass out or lose the ability to perform basic tasks. Given the impossibility of providing an oxygen system for every passenger, the engineers decided that the best solution was to take the TOC and instruct the pilots to dive to a breathable altitude in that amount of time.
That would be no big deal except the TOC is something like 90-120 seconds at Concorde's 60,000 foot cruising altitude. A breathable altitude without supplemental oxygen is around 15-20,000 feet. So in the event of a loss of cabin pressure, the emergency procedure on the Concorde was to dive 40,000 feet in 90 seconds. As far as I can tell this maneuver was never actually tested, although engineers did determine that it was well within the structural limits of the airframe. While the aircraft itself might have survived, I highly doubt the same could be said for the structural integrity of the passengers' pants if this actually happened on a revenue flight. Given the high cost of tickets and the high-class clientele that the Concorde attracted, explosive pant-soiling would have been quite the public relations nightmare for British Airways. Thank god it never came to that, and thereby were many pants saved.