In this sense, it is unsurprising to point out that these collections of marks-on-the-world tend to congregate and compile in the city's underground. Some of this is due to drift, or the amount of daily time we spend under the street here and in other metropolitan areas around the world, but I believe some of it is a hold over from supposedly long-lost instincts. From the caves at Lascaux to Plato's allegory of the cave, enclosed spaces have held a fascination for our creative impulses since our first recorded expressions. In many ways it just makes practical sense - underground is generally considered a safe repository, as far as global environments go.
What can be surprising is when the underground serves as an archive or record without conscious effort. In many ways, we can uncover a strange archival topography underground, just by remaining observant. One of my favorite pastimes while waiting for the train is spotting remnants of the The French Connection New York amidst all the now-a-days You've Got Mail glitz. (Or for that matter, spotting the Taking of Pelham 1-2-3 underneath all the Taking of Pelham 1-2-3.) Whether you're a New York diorama artist creating a personal quasi-geological record of Canal Street, or simply a vandal exposing last week's new release under this week's newly-plastered poster, the layers of the underground are too many and varied to count.
Sometimes, whole rooms preserve whole eras. This can give us a glimpse of turn-of-the-century architecture, or even help us pinpoint the frozen year when a section was converted into unintentional archive. The possibilities are pretty endless, and who can say? In a few hundred years, our means of commute might prove our most extant public record for anthropologists. So I say: Keep making your mark.