Scientists say coral bleaching is getting worse in southwestern Japan.
It is feared the damage, which includes areas near the Island of Okinawa, could be the most serious in nearly 20 years.
Water temperatures have topped 30 degrees Celsius this year. Under such conditions, coral bleaching usually takes place within two to four weeks.
A lack of typhoons in the region is one explanation being put forward by scientists for the unseasonal high water temperatures.