Clay Kickers is a set of free wargames rules for tunnel warfare in World War I. The authors write:

Ypres, 1917. A great battle rages overhead, the like of which the world has never seen before. An unceasing exchange of industrial Firepower, grinding men into ash and dust. Beneath the earth, a different kind of struggle is waged. Men cram themselves into lightless burrows, undermining the great earthworks above with great quantities of explosives. This is the story of that struggle.  Clay-Kickers is an objective based skirmish wargame for two players. Each player controls a squad of engineers, demolition experts and battle-hardened close-quarter blighters. With their motley crew of tunnel rats, the players will create their own tunnels under the battlefields of the Western Front, avoiding underground hazards and engaging the enemy in brutal close-range brawls.