A fierce snowstorm strikes the treacherous Himalayan mountain. The freezing wind is so strong the climber can barely stand without being blown away.
The swirling snow has blurred her vision and the biting cold - the temperature is about minus 20 degrees Celsius - leaves her fingers numb. Tightly grabbing her climbing rope fixed in place by her Nepalese guide, she takes the next step. Before she even realises that anything is wrong, half of her body is devoured by the impenetrable darkness of a bottomless crevasse.
All that is supporting her is a rope bolted in the snow dangerously close to the chasm. Her life is literally hanging by a thread.
Four years ago, Ada Tsang Yin-hung promised her pupils she would climb to the top of the highest peak in the world. At that time, she had no experience of mountaineering at all.
"I was setting life targets with them," says Tsang, who teaches life education at CUHKFAA Chan Chun Ha Secondary School in Ma On Shan. "Many people don't dare to tell others what they want to achieve because these dreams seem to be too unconventional or impossible. I want to set a completely outlandish target for myself and let my pupils know my progress. I want to show them that if I can do the seemingly impossible, so can they."
Tsang came up with the idea of tackling the 8,848-metre Mount Everest after two previous cycling treks on the Tibetan plateau. In 2008, she and three of her pupils cycled along the 4,000-kilometre-plus Sichuan-Tibet Highway, known as one of the world's highest and most treacherous roads. Two years later, she took off with another three pupils for a cycling trip from Lhasa in Tibet to Urumqi , capital of the Xinjiang autonomous region.
She was mesmerised by the breathtaking views of the snow-covered mountains en route and decided she would one day get closer to the pristine peaks.
To be true to her word, Tsang learned mountaineering on Mount Siguniang in Sichuan over the following three winters, and reported her training to her pupils in detail so that they could keep track of how she was moving, step by step, towards her goal.
With the children's support and HK$400,000 raised from people and organisations affiliated with her school, Tsang finally set off in April, forming a team with three women from the mainland and their Sherpa guides to conquer Everest.
But the team was forced to give up after an avalanche on Everest that month killed 16 Sherpas and blocked the only path from the south to the summit.
When disaster struck, she was training on another peak.
"We felt really sympathetic for our Sherpas because we knew some of their friends died in the avalanche," Tsang says. "But we didn't plan to give up. On the contrary, we felt relieved because we thought it would be even safer to climb Everest after the unstable ice block fell."
They did not feel disheartened until they reached South Everest base camp 5,364 metres up, where they learned that two-thirds of the 36 expedition teams had left or were leaving.
"We were waiting anxiously at the camp when the Sherpas were meeting to decide if expeditions should continue. Everyone was hanging on to the last thread of hope that we could go on."
They eventually left on May 4, following an April 22 announcement that the Sherpas would not work on Everest for the rest of the year out of respect for the victims.
But they then decided to take on another peak, the 6,187 metre-high Pharchamo in the same mountain range. After days of hiking along cliffs, snow-laden slopes and rocky paths, they reached the peak's base camp at an altitude of 5,700 metres. At that point, all that stood between them and success was a seemingly easy 487-metre section of the path to the top.
But that short journey almost cost them their lives as potentially lethal weather rolled in.
It was there that Tsang fell into the crevasse. Her backpack tangled with her safety rope, preventing her from pegging her axe into the glacier to stabilise herself.
"For the first time I was in panic," Tsang says. "I had seen it happen in films but I never thought the victim would one day be me. All the tales about people falling into the centre of the earth suddenly came back to me."
Thankfully her Sherpa arrived quickly, calmed her down and guided her in disentangling her backpack from the rope and finally pulled her up. Tsang managed to continue. When the team reached the peak, they only stayed for a short while - not least because they had to crawl on all fours to avoid being blown away.
All Tsang wanted to do was tell her pupils she would not give up.
She plans to return to Everest in April next year to complete her unfinished mission. She has also set a new target for the next expedition: she plans to conquer the 8,516-metre-high Lhotse, the fourth highest mountain on earth, right after reaching the top of Everest.
"I want to tell my pupils that when you've reached what you can reach, you can achieve more," she says. "Every achievement can inspire you to take the next step."
It is almost as if she is climbing not for herself, but for her pupils. In fact, besides mountaineering, she has taken up basketball, cycling, kayaking, trekking, camping and orienteering, all because she wants to engage different pupils in activities they enjoy.
"My pupils have pushed me to learn a lot of things," she says. "If you suck in these activities, your pupils won't appreciate you. Teenagers only listen to those they admire, and I want to be their hero."
She also finds outdoor activities an effective way to teach children how to deal with challenges and engage them in study.
She once took her pupils to visit Mui Wo in the afternoon and do their homework together at the pier, and she has toured many mainland cities with her pupils.
She also took two pupils to do volunteering work in the days when they should have been preparing for their exams. The two pupils exceeded expectations in the exams.
Tsang says parents do not understand that there are different ways of engaging children. Sometimes, even her school did not quite understand and would stop extracurricular activities before exams.
"It's a completely suicidal decision," says Tsang. "If you don't even give them a chance to move, where can they get the motivation to study?"
1999: Graduates from Chinese University with a bachelor's degree in environmental science
2000: Graduates from the University of Hong Kong with a master's in education
2000-2003: Teaches maths and integrated science at CUHKFAA Chan Chun Ha Secondary School in Ma On Shan
2004-2006: Obtains master's in environmental management from Queensland University
2006-2008: Teaches biology, integrated science and integrated humanities at Chan Chun Ha
July to September 2008: Cycles from Sichuan to Tibet
2008-2010: Volunteers at organic farm in Guangxi
July to September 2010: Cycles from Lhasa to Urumqi
2010-present: Teaches life education, maths and liberal studies at Chan Chun Ha
April 11, 2014: Sets off for Everest expedition
May 27: Returns to Hong Kong