Externalities, as defined elsewhere, is the shifting of costs from the buyer-seller pair to a third party. Externalities are a form of market failure — when the pricing mechanism fails to capture the full and true cost of the product or service.
The U.S. Superfund Sites list, is the list sites requiring a toxic contamination cleanup. Typically, these sites are the where now defunct manufacturing businesses operated in the days when “dumping out the back of the factory” was the cheapest toxic chemical disposal method. The sites are contaminated beyond safe use — and they often pose hazards to people downwind or downstream of them — until they are cleaned up (remediated).
Under the US law creating the Superfund, polluting companies are responsible for the cleanup costs. If the companies are bankrupt or no longer exist then US taxpayers pay for the cleanup. In the former case, that is shifting externality/costs from prior stockholders of the polluting company to current stockholders. In the later case, it is shifting externalities/costs from the stockholders of the polluting company to today’s US taxpayers.
As of 2014, there were 1,322 sites on the list; 53 more proposed additions; and 375 sites removed from the list because they have been remediated and do not require on-going management (source).
This National Geographic interactive site lets you find Superfund sites near you.
More broadly, the ToxMap site plots a broader list of contaminated toxic sites — it is largely a map of urban populations, railroads, and US highways but with some notable “off the beaten path” exceptions.