From Bulbapedia, the community-driven Pokémon encyclopedia.

Mt. Battle (Japanese: バトル山 Mt. Battle) is a large mountain in the north of Orre renovated to be the ultimate challenge for Pokémon Trainers. Challengers must battle through ten areas, each containing ten trainers, to pass the challenge-- a total of one hundred battles.

The mountain itself is exceedingly large, and has a variety of terrain, ranging from basic rocky crags, to the volcanic inside of the mountain, to the lofty peaks. The pathway up Mt. Battle is made with metallic platforms suspended in mid-air with propellers.

In Pokémon Colosseum, after completing the game and purifying all Shadow Pokémon, Ho-Oh can be found and captured at the top.

Challengers who pass through areas win Poké Coupons they can exchange for prizes, such as TMs and rare items.

Challengers who wish to quit or rest are teleported to the foot of the mountain thanks to a series of friendly Abra that are enlisted as part of the staff.

Trivia

In Pokémon XD: Gale of Darkness, the trainer on platform 35 states "I've heard someone has the same team combination as me. Do you know him or her?". With a team consisting of Cacnea and Chimecho, the person with the same team combination is obviously James of Team Rocket.