  To perform a longitudinal investigation of web archives and detecting
variations and changes replaying individual archived pages, or mementos, we
created a sample of 16,627 mementos from 17 public web archives. Over the
course of our 14-month study (November, 2017 - January, 2019), we found that
four web archives changed their base URIs and did not leave a machine-readable
method of locating their new base URIs, necessitating manual rediscovery. Of
the 1,981 mementos in our sample from these four web archives, 537 were
impacted: 517 mementos were rediscovered but with changes in their time of
archiving (or Memento-Datetime), HTTP status code, or the string comprising
their original URI (or URI-R), and 20 of the mementos could not be found at
all.
