SAN FRANCISCO INTERNATIONAL (CBS SF) – Two plumbers were rushed to the hospital after a carbon dioxide canister ruptured inside an underground tunnel near San Francisco International Airport, leaving one unconscious and the other scrambling to save his co-worker’s life.

It happened around 4 p.m on Saturday. Paramedics, firefighters and emergency crews arrived in a matter of minutes.

According to airport officials, the two were cutting concrete under a building located at 649 Field Road, which is the prep and packaging building just north of the airport, when a carbon dioxide canister ruptured releasing toxic fumes.

One man was knocked unconscious by the fumes. Then, the other man tried to drag the unconscious man to safety, but had to leave him behind in the maze of tunnels. When he returned with a rescue team, the man was gone.

Officials described a very complex network of underground passages which made the search quite challenging.

“The building footprint underneath is large, there are multiple rooms, also tight spaces so everybody has to use abundance of caution,” said Airport Duty Manager Russell Mackey.

Officials determined the air inside the tunnel was safe to breathe and crews worked for two hours to locate the second plumber.

Firefighters turned every corner and looked into each room with no luck at first. Then the tricky search came to an end as the plumber was found.

According to Mackey, the man said he regained consciousness and was looking for an exit when the rescue team found him.

Both men were transported to the hospital by ambulance.

No word on their condition, or what caused the canister to explode.