The Himalayas was a mountain range in the middle of the continent of Asia on Earth.
Susan Foreman believed that she, the First Doctor, Ian Chesterton and Barbara Wright had landed in the Himalayas when the TARDIS once abruptly malfunctioned. She called the Himalayas the "Roof of the World". Exploring, the party met Marco Polo, who told them that the year was 1289 and they were not in the Himalayas, but the Plain of Pamir, an area that he called the "Roof of the World". The TARDIS crew was invited to join him on his trek to Shang-Tu, the summer palace of Kublai Khan. (TV: "The Roof of the World")
In 1917, the Fifth Doctor took Erimem to Tibet for a legendary cricket match, only to unwittingly plunge her and Peri Brown into a confrontation with the Great Old Ones. (AUDIO: The Roof of the World)
In 1935, the Second Doctor, Jamie McCrimmon and Victoria Waterfield went to the Himalayas for a holiday at the Det-Sen Monastery in Tibet, only to find it was under attack by robot Yeti crafted by the Great Intelligence. (TV: The Abominable Snowmen)
Torchwood India later retrieved several Robot Yeti control spheres from the Himalayas. (AUDIO: Golden Age)
In 2008, the Saxon Master (under the guise of Harold Saxon) sent the Torchwood Three team on a "wild goose-chase" to the Himalayas to prevent Jack Harkness contacting his team for help resisting the Master. (TV: The Sound of Drums)
Behind the scenes
"The Roof of the World" gives two different locations for the "Roof of the World". Susan maintains the Himalayas as the "Roof", while Marco Polo clearly states that the Plain of Pamir is. Though unexplained by the episode or the novelisation, they're both right. The Pamir Mountains are said to be at the intersection of the Himalayan, Tian Shan, Karakoram, Kunlun and Hindu Kush ranges. Thus, in a sense, Susan is right, but the "Plain of Pamir" is a more precise location.