In 1972, in a mosque in the city of Sana'a, Yemen, manuscripts were discovered that were later proved to be the most ancient Quranic text known to exist at the time. The Sana'a manuscripts contain palimpsests, a manuscript page from which the text has been washed off to make the parchment reusable again—a practice which was common in ancient times due to scarcity of writing material. However, the faint washed-off underlying text (scriptio inferior) is still barely visible and believed to be "pre-Uthmanic" Quranic content, while the text written on top (scriptio superior) is believed to belong to Uthmanic time. Studies using radiocarbon dating indicate that the parchments are dated to the period before 671 AD with a 99 percent probability.