Big deck is a format primarily played in Alberta. It is similar to the format Battlebox https://luckypaper.co/articles/a-guide-to-battle-box/

Players begin by grabbing an arbitrarily large pile of cards from the card pool. That is your library for the game and is replenished as it runs low.

On your turn, you may play any card face-down as a land. This land taps to produce one mana of any type, , , , , , , .

Face-down lands have no subtype or supertype associated with them. E.g., they are not Forests, Basic, non-Basic, Legendary, and have no name.

If a card instructs you to shuffle a card back into your library, or put a card onto the bottom of your library, it is instead put in the pile of already used cards from previous games. This is not a special zone and cannot be interacted with further. Cards "removed from the game" in this way are not public information. For example, if you Scry 3, and put two cards on the bottom of your library, these cards are instead removed from play, but opponents may not see what they are.

If a land moves to any zone except exile, it stops being a land and returns to being the face-up card. For example, if you play Deep Analysis as a land, and it later gets destroyed, it will be a copy of Deep Analysis in the graveyard and may be flashed back. In contrast, if you play Emrakul, the Aeon's Torn face down and it gets targeted by Parallax Tide, when the Parallax Tide leaves the battlefield, it will return to play as a land.

If a player would fail to find a card when searching, they are permitted to continue searching through unused portions of big deck and add it to their library. However, in that case they must stop at the first legal choice. A player may only continue searching the first time they search with a card, and this restriction persists for the rest of the game.

For example, if a player searches their library with Primeval Titan, and would fail to find two lands, they may add more cards to their library, but must stop searching as soon as two lands are found. They may not add any more cards if they later attack with Primeval Titan and fail to find lands again, even if that card left play due to Momentary Blink and the game rules consider it a new object.

Playing a card facedown as a land is a special action and does not otherwise interact with cards like Crucible of Worlds or Uro, Titan of Nature's Wrath, but does interact with a card like Explore.