The close value of the S&P 500 on the date this market opened was 4,283.74. This market resolves YES if the S&P 500 closes at less than 42.84 on September 12 (a 99% drop). Otherwise resolves NO.

(Note that if the S&P drops below this threshold on a date before September 12, that does not cause a YES resolution).

Meta comment: This is intended as both as a market on black swan events and also as a comparison against similar "risk free interest rate" markets. None of them are entirely risk-free (e.g. author might misresolve, or author might get hit by a bus leaving the market abandoned unresolved), so the question is how much different types of risk empirically matter to market participants. Paired question:

Aug 20, 7:05pm: corrected 428.37 to 42.84.