I did find the following relating to a fire in a tunnel - French, not German thoughWerent a majority of the Germans entombed there due to IIRC cooking rations too close to flamethrower fuel and Handgranate?
"Early in the battle the French lost the great fort of Douaumont, which has served the Germans as a secure forward base since then. On the French side, a similar role is served by the Tavannes Tunnel. Running under the last line of fortifications before Verdun, this railway tunnel has become a field hospital and forward shelter from German artillery.
Unfortunately for the French, the same kind of disaster strikes the French in the tunnel as befell the Germans in Fort Douaumont in early May. Rockets being carried into the tunnel by mules are somehow ignited. They set off a chain reaction, detonating grenades, more munitions and inflammable fuel stores. The result is a series of explosions and then an uncontrollable fire, with men killed by flame or blast, or trampled to death as they attempt to flee to safety. More than 500 men lose their lives in the inferno."
From Battle of Verdun – World War 1 Live
There is also information relating to the Gallwitz tunnel, which was unfinished, I have been down into it through a ventilation shaft but only to the first level (ventilation corridor), as the tunnel is sealed there is a high level of probability of dead air, the AQM was flashing a warning and we didn't have caving BA with us so chose to go no further.
VERDUN. Gallwitz-Tunnel auf Mort Homme. Toter Mann (in German)
A good account of the battles of Verdun here The Battle of Verdun 1916 - the greatest battle ever (in English)