Blue Hole In Gaum is the important island. Guam is the southernmost island in the Mariana island chain and the largest island in Micronesia. Guam's Blue Hole is a natural limestone vertical shaft formed millions of years ago which extends down into the reef. The opening is approximately 35 x 25 feet. The upper reef varies in depth from 20 feet near the cliffline, to 55-60 ft at the upper opening of the shaft. From there, it drops downward to 300+ feet.