The Iranian rial slumped today to a historic low against the U.S. dollar. According to Iranian media and currency tracking websites the rial fell by 3.6 percent and was trading at about 27,900 per dollar. Persian-language currency tracking website Mazanex says,there is no clear reason for the latest slide in the value of the rial, which is traded in an informal market of money changers, where ordinary Iranians go to access hard currencies. Previous falls have coincided with new sanctions or diplomatic isolation of Iran over its contested nuclear program. In June last year, it was trading at about 12,000 to the dollar, or more than twice its value now.