More than 20 people may be missing or unaccounted for following flash flooding that's closed several popular parts of Grand Canyon National Park in Arizona."The National Park Service is asking anyone who knows of hikers or backpackers who were in the inner canyon along the Bright Angel Creek corridor on August 29, as well as anyone who had a campground reservation in the affected corridor, to provide information to the National Park Service Investigative Services Branch (ISB)," according to a park news release. "This includes individuals who may have been traveling along the North Kaibab Trail from the North Kaibab Trailhead to Phantom Ranch, staying at Bright Angel Campground, or traveling or staying in the area known as 'the Box' north of Phantom Ranch."The park shared the tip line 888-653-0009.Flooding in the area began Aug. 29.As of early Aug. 30, more than 60 people had already been evacuated and more evacuations were scheduled, according to a previous news release.The park said more flash floods and hazardous conditions were possible over the next two days, amid potential thunderstorms and downpours.What's so grand about Grand CanyonAs Grand Canyon National Park celebrates its centennial, here are some awe-inspiring figures about one of the most recognizable landmarks on the planetMark Henle, USA TODAYWhat parts of Grand Canyon are closed?The park said the following areas are closed until further notice, as staff assess damage:Bright Angel CampgroundPhantom Ranch, Phantom Ranch Canteen and cabinsThe entire North Kaibab Trail from North Kaibab Trailhead to Phantom RanchBlack Bridge and Silver Bridge crossing the Colorado River"There is currently no estimated reopening date for these areas and other inner canyon closures may be necessary," the park said, adding that nearly all footbridges across Bright Angel Creek have been destroyed and the Colorado River is closed to river traffic due to debris in the water.Visitors with upcoming river trips are advised to contact their respective outfitter or NPS River Permits. Visitors with Phantom Ranch reservations will be notified by park concessionaire Xanterra.All visitors should check the park's Key Hiking Messages and Alert & Conditions webpages for the latest information.Contributing: Jose R. Gonzalez, Arizona Republic