CMC: 0 Korttyp: Land -- Island Nuvarande oracletext: (Tap: Add U.) Mystic Sanctuary enters the battlefield tapped unless you control three or more other Islands.
When Mystic Sanctuary enters the battlefield untapped, you may put target instant or sorcery card from your graveyard on top of your library. Illustration: Randy Vargas Kortspecifika regler
An Island is a land with the subtype Island, not any land that could produce blue mana. For example, Mystic Sanctuary is an Island, but Castle Vantress isn't.
As these lands are entering the battlefield, they check for lands that are already on the battlefield. They won't see lands that are entering the battlefield at the same time (due to Scapeshift, for example).
If another effect puts these lands onto the battlefield tapped, they enter tapped, even if you control enough lands with the appropriate basic land type.