Complex sentence: There have been over three hundred sightings of boys matching Ben's description reported, both on the Greek mainland and on Greek islands, most called in shortly after his disappearance during the period of 1991 to 1992.
Split into simpler sentences: There have been over three hundred sightings of boys matching Ben's description reported, both on the Greek mainland and on Greek islands. Most were called in shortly after his disappearance during the period of 1991 to 1992.