version 518, map tiles downloaded from the Internet are held in a
cache. This improves access speed when scrolling around the map or
zooming in and out.
By default the cache is cleared after each session but a this can be
changed with a checkbox on Edit>Preferences and Settings General
Tab. Then the cache is preserved between sessions. Each time the
program starts, it displays a summary of the cache contents by number
of days since last access. The cache will grow indefinitely - but
hitting F2 will clear the oldest day's files.