Manholes can be found in Epic Mickey 2: The Power of Two and Epic Mickey: Path Painter.
Epic Mickey 2: The Power of Two
Manhole covers lead to the D.E.C.. They appear in a variety of places, mostly before the Projector Screens are repaired.
Epic Mickey: Path Painter
Manholes automatically respawn Spatters continuously. They can be thinned over to prevent anymore Spatters appearing.