Standard chess
Movement, captures, check/checkmate, castling and en passant follow standard chess. On promotion, choose Queen, Rook, Bishop or Knight.
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Underground Chess Rules
Everything you need to know before starting an Underground Chess game.
Two tactical layers
Every piece starts on Surface and may exist on either Surface or Underground within the same 8×8 coordinates.
Bidirectional tunneling
Tunneling down and back up are both legal; a piece may capture an enemy on the matching square of the other layer.
Limited tunneling
Each side always has at least one free tunnel; the maximum tracks surviving pawns across both boards, with no normal tunneling on consecutive turns.
Knight breaks depth
Knights move or capture in an L across layers without limit and without consuming normal tunnel quota.
Free response
Escaping check by tunneling gives the opponent one tunnel response on the next turn without consuming free quota.
Chess clock
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
Take back is available only against the computer or in Practice mode. Normal two-player competitive games do not allow it.