Archaeologists at Mesa Verde National Park say a crack is threatening the southern half of the park's largest and most famous cliff dwelling.The park announced Friday that the crack and other related structural problems is the reason why visitors have been kept away from the edge of a kiva on tours of Cliff Palace this summer. Wooden braces are now shoring up the kiva, a round, Pueblo Indian ceremonial structure built in the 13th century.The crack was discovered last summer but it's not the only problem archeologists are working to fix. Cliff Palace was built on a sloping alcove floor and, over time, it has been sliding downslope. Dripping water has been a long-standing problem too but water is now being rerouted away from the building.The National Park Service said a fault line is running parallel to the back of Cliff Palace and is threatening Kiva F and the southern half of Cliff Palace.Cliff Palace is visited by approximately 160,000 visitors per year. The dwelling was discovered in the late 1800s and remains one of the finest examples of late prehistoric cliff dwellings in the American Southwest.Park archeologists first noticed a small crack in the masonry and then discovered the north wall was leaning precariously into the structure. After months of study, it now appears that the localized deterioration of individual structures and features and site-wide problems are working together. Much of the architecture in the southern portion of Cliff Palace is footed onto small, irregularly shaped roof slabs across a steeply pitched alcove floor. Over time, these structures have begun to slide downslope. Water entering the alcove through cracks on the mesa top above Cliff Palace has also been a long-standing problem.Cliff Palace remains open for tours while restoration plans are underway and runoff water is rerouted. Archeologists have roped off Kiva F to tours and continue to evaluate and monitor the site.Cliff Place is important to the history of the area, its a sacred site for our tribes and its important to the economy of the area as well. said Mesa Verde National Park Superintendent Cliff Spencer. We are working toward a solution to stabilize Kiva F and Cliff Palace itself.For further information on Cliff Palace and Kiva F, please visit our website at www.nps.gov/meve.