Despite a perilous, unstable ice block hanging over a key trail on Mount Everest, and undeterred by high costs and increased permit fees, hundreds of climbers are pressing ahead with their ascent of the world's highest mountain. Around 410 climbers and an equal number of Nepali guides are currently at base camp, preparing for their attempt on the nearly 8,850-metre (29,000-foot) peak this month, hoping to capitalise on a narrow window of favourable weather. Climbers gathered last month at the 5,300-metre (17,340-foot) base camp, but a massive, unstable ice block, or serac, stalled progress for over two weeks. The "Icefall doctors" – elite guides from the Sagarmatha Pollution Control Committee (SPCC) who lay the annual route with ropes and ladders – usually finish by mid-April, but were delayed this year. The Icefall route was only opened on 29 April, accompanied by a severe caution. The SPCC warned: "The serac has multiple cracks and may collapse at any time.…