  Purpose: To provide a critical review of Bergman's 2001 study on the Deep
Web. In addition, we bring a new concept into the discussion, the Academic
Invisible Web (AIW). We define the Academic Invisible Web as consisting of all
databases and collections relevant to academia but not searchable by the
general-purpose internet search engines. Indexing this part of the Invisible
Web is central to scientific search engines. We provide an overview of
approaches followed thus far. Design/methodology/approach: Discussion of
measures and calculations, estimation based on informetric laws. Literature
review on approaches for uncovering information from the Invisible Web.
Findings: Bergman's size estimate of the Invisible Web is highly questionable.
We demonstrate some major errors in the conceptual design of the Bergman paper.
A new (raw) size estimate is given. Research limitations/implications: The
precision of our estimate is limited due to a small sample size and lack of
reliable data. Practical implications: We can show that no single library alone
will be able to index the Academic Invisible Web. We suggest collaboration to
accomplish this task. Originality/value: Provides library managers and those
interested in developing academic search engines with data on the size and
attributes of the Academic Invisible Web.
