Very often, archaeological remains take a form that most folks (and even most archaeologists) never think of. And this can create odd situations and interesting opportunities.
Case in point. I have been working on a historic-era archaeological site the is comprised of the remains of an old manufactured gas plant. Manufactured gas is a product of coal*, where the coal was heated in oxygen-poor environments to release combustible gases, which, during the 19th and early 20th centuries, were piped to street lamps and homes to provide fuel for gas lamps.
The production of manufactured gas results in the production of a range of waste products, most of them toxic to some extent. Given that manufactured gas was primarily used during the 19th and 20th centuries, the waste was not typically disposed of in an environmentally safe fashion. The end result is that former manufactured gas facilities are very often waste sites, and that the companies (often, though not always, public utilities) that own them are now saddled with the responsibility of cleaning up the mess made by previous generations.
Because these plants are typically well over 50 years old, and the National Historic Preservation Act (and, in California, the California Environmental Quality Act) require the evaluation of archaeological sites over 50 years in age, it is common for archaeologists to be present monitoring the clean-up and remediation work done at these sites. Which brings us to the interesting point - the chemical waste itself is a product of the use of the manufactured gas facilities that comprise these sites, meaning that the chemical waste is an archaeological deposit.
No sensible person would recommend leaving these waste materials in the ground, and so I am not advocating that they be protected under the relevant regulations - nor would they be, as safety regulation typically trumps historic preservation. But, as I have been doing this work, I have found myself taking much more extensive notes than I typically would for a monitoring project. Given that the waste will eventually be removed and processed, the monitoring of the clean-up provides a weird opportunity to fully document the archaeological site as it is dismantled for proper waste treatment. In fact, as some of these sites represent the intact remaisn of the disposal of waste from the manufactured gas plants, they are intact features by any reasonable definition.
Of course, few people, including few archaeologists, think of these materials this way. This is usually considered nothing but toxic waste, and the notion that it represents and intact archaeological deposit, devoid as it is of tools and construction materials, would strike most as odd. But, well, there you have it.
Okay, not the most exciting archaeological site ever, and probably not that interesting to most people. But these are the sorts of things I think while I stand around and watch people produce soil cores. I have to convince myself that there's a silver lining some times.
*Other combustible materials, such as wood and oil, could also work, but it was almost always made from coal.