  Search engines provide cached copies of indexed content so users will have
something to "click on" if the remote resource is temporarily or permanently
unavailable. Depending on their proprietary caching strategies, search engines
will purge their indexes and caches of resources that exceed a threshold of
unavailability. Although search engine caches are provided only as an aid to
the interactive user, we are interested in building reliable preservation
services from the aggregate of these limited caching services. But first, we
must understand the contents of search engine caches. In this paper, we have
examined the cached contents of Ask, Google, MSN and Yahoo to profile such
things as overlap between index and cache, size, MIME type and "staleness" of
the cached resources. We also examined the overlap of the various caches with
the holdings of the Internet Archive.
