stinky tunnel

a few months back i noticed a fart-like smell when exiting the tunnel going from berkeley to orinda. i just assumed someone in my car dropped ass. but almost everyday since then i have noticed this same smell at the same spot on my commute. i assume it is possible that there is a fellow passenger with IBS that executes a well timed fart everyday to avoid suspicion. more than likely, however, there is some kind of leak near the track. doesn't smell like sulfer or natural gas. it smells exactly like a fart. does anyone know what this is? i smell it exiting the tunnel that parallells the caldecott on the orinda side in the evenings, and occasionally in the mornings when entering the tunnel from the orinda side. extra credit if you know the name of this tunnel.