The funny thing is you never really encounter real darkness in the surface world. I kept waiting for my eyes to adjust and start to discern details, like they would in an ordinary darkened building, but they never did. It was still pitch black after 20 minutes. Meanwhile, we kept still and listened to sounds from the outside.
Despite being underground and surrounded by concrete, a fair bit of outside sound managed to seep in. After maybe 10 minutes we began to whisper to each other. What I heard as a tree limb creaking, my late trubbamaking companion thought was a woman's voice echoing through the cement complex. It sounded like soft speech at first, then gentle moaning. The constant creaking of tree limbs from from above had already suffused our whole evening with tension, but the thought of a couple of Muni employees sneaking into the tunnels behind the storeroom to have sex didn't seem too farfetched. Meanwhile I kept imagining I heard single, deliberate footsteps, just on the edge of audibility, somewhere out in the passageways.
After 20 minutes we got tired of waiting and cautiously found our way back out of the complex, through the dense walls of ivy again, and climbed the tangled vines back up to ground level. As a final added scare, just as I reached the surface, the tree branch that we had heard "moaning" from so far below let out a sudden loud creak at my elbow, and I nearly lost it.