The phrase "ice mountains" refers to very tall and large formations made of ice. It means mountains covered in ice instead of earth or rocks. Full definition
If you like taking the refreshment with you no matter where life takes you, then you'll appreciate the many sizes in which Ice Mountain ® Brand Natural Spring Water is available. (nestle-watersna.com)
The discovery of ice mountains on Pluto and water on Mars? (automobilemag.com)
LAUREL, Maryland — After 85 years as a mystery, the surface of Pluto is finally coming into focus, with a new NASA photo revealing towering ice mountains rising from its surprisingly youthful face. (space.com)