A massive ice block on the route above Mount Everest’s base camp is delaying climbers seeking to scale the world’s highest peak. An aerial survey is planned to evaluate the serac that’s between base camp and Camp One. Mountaineering officials said Friday the serac was unstable and risky to climbers. It is located in the Khumbu Icefall, a difficult part of the mountain that is known for its shifting glacier and deep crevasses. Climbers and guides are waiting at Everest base camp as officials assess the situation. More than 400 foreigners have been issued permits to climb Everest during the spring climbing season.