Crescent Island is a sunken island that first appeared in "Appetite for Destruction".
Covered with sparse foliage like evergreen trees and bushes, this island is relatively ordinary looking. It is littered with relatively large rocks and a few boulders.
In "Appetite for Destruction", a dragon migration survey brings Tuffnut and Ruffnut to Crescent Island, where they tag dragons with purple paint. Unsurprisingly, they get distracted. Later, the Screaming Death sunk the island when it was heading to Berk, causing the dragons there to flee towards areas like Thor Rock Island.