The phrase "treacherous mountains" means that the mountains are very dangerous and could be risky or deceitful to those who try to navigate or explore them. Full definition
This unit assists hikers through the hundreds of treacherous mountains in the country to try to ensure their safety. (animalleague.org)
After a rest, Shackleton and two of his men hiked and climbed across treacherous mountains to a whaling station, where Shackleton procured a ship and sailed to rescue his comrades. (cnbc.com)
The highly anticipated Spectre, the last installment of the 007 franchise for actor Daniel Craig, delivers jaw - dropping scenes of James Bond scaling a rooftop and skiing through treacherous mountain passes. (travelandleisure.com)