A charity climber developed hypothermia when he tackled Snowdon wearing just his underpants.
19-year-old Nathan French wore Superman underwear when he climbed the mountain - the highest peak in England and Wales - in aid of a dementia charity, the Daily Telegraph reports.
He became unwell because of the cold and required medical treatment. Now mountain rescuers are urging other climbers to make sure they wear appropriate clothes.
Phil Benbow, of North Wales Mountain Rescue Association, told the BBC that a "significant number" of calls were received each year from people who are not equipped properly.
"They make an assumption that because there's a cafe at the top and a train, that it's a walk in the park. It's not - it is the highest mountain in England and Wales and people need to respect that.
"Our team has been called to 173 rescues so far this year and that will be over 200 by the end of the year - the third year in a row we will have topped 200."
Mr French said he was surprised by "how cold I got".
"I was shaking uncontrollably and they covered me in tin foil," he said.
"On the train down I started to feel really sick and I started going deaf and my eyesight started going funny. I started getting really emotional so my dad, who was with me, decided to call an ambulance."
Safety advice from Snowdonia National Park Authority includes checking the weather forecast before setting out and wearing waterproof, windproof, warm clothes.
Mr French raised nearly £1,000 from the climb.