These translucent bowls require time to make but need very little attention, and can be made a day or two in advance.
Cut tops off grapefruits, a third of the way down from stem end. Place trimmed bottoms in 12-quart stockpot; cover with 4 quarts water. Bring to a boil, reduce heat, and simmer, uncovered, 2 hours.
Remove grapefruits; place upside down on a wire rack to drain. When cool enough to handle, scoop out flesh with a spoon (do not puncture grapefruit skin).
Combine sugar, corn syrup, and 5 cups water in the stockpot; bring to boil over medium heat, using wet pastry brush to brush down sugar crystals that form on side of pot. Clip candy thermometer to pot; cook until syrup reaches thread stage (230 degrees). Add grapefruit bowls, reduce heat, and simmer at least 1 hour, occasionally turning gently. Grapefruits should be unevenly translucent. Turn off heat; let cool in syrup until completely translucent. Using whisk small enough to fit in bowls, gently lift them from syrup. Place upside down on rack over baking sheet. Let dry overnight. Store in airtight container lined with parchment paper.
Use the syrup to sweeten drinks, or reduce it and use as a glaze.