A botched experiment in a tunnel in which water, seeping into its cracks and folds, created a lapping sound, not dissimilar to the slap and slosh found on beaches. Down the street a BGE truck pumped a steady hum, which the tunnel captured quite eloquently.
It's short in duration because the mics captured more hand-interference than environment. If I had a picture of myself hanging (almost) upside down, it could have been more interesting than the recording.
Next time, bring the boom!