You can choose to find any card with the Mountain land type, including nonbasic lands. You can also choose not to find a card, even if there is a Mountain card in your library.

Unlike the normal cycling ability, Mountaincycling doesn't allow you to draw a card. Instead, it lets you search your library for a Mountain card. After you find a Mountain card in your library, you reveal it, put it into your hand, then shuffle your library.

Mountaincycling is a form of cycling. Any ability that triggers on a card being cycled also triggers on Mountaincycling this card. Any ability that stops a cycling ability from being activated also stops Mountaincycling from being activated.

Mountaincycling is an activated ability. Effects that interact with activated abilities (such as Stifle or Rings of Brighthearth) will interact with Mountaincycling. Effects that interact with spells (such as Remove Soul or Faerie Tauntings) will not.