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Underground Chess rules

Everything you need to know before starting an Underground Chess game.

Movement, captures, check/checkmate, castling and en passant follow standard chess. On promotion, choose Queen, Rook, Bishop or Knight.

Every piece starts on Surface and may exist on either Surface or Underground within the same 8×8 coordinates.

Tunneling down and back up are both legal; a piece may capture an enemy on the matching square of the other layer.

Each side always has at least one free tunnel; the maximum tracks surviving pawns across both boards, with no normal tunneling on consecutive turns.

Knights move or capture in an L across layers without limit and without consuming normal tunnel quota.

Escaping check by tunneling gives the opponent one tunnel response on the next turn without consuming free quota.

Each side has an independent clock from 1 to 120 minutes. Leaving it blank defaults to 30 minutes; running out of time loses the game.

Take back is available only against the computer or in Practice mode. Normal two-player competitive games do not allow it.