Blue holes occur at Cockatoo and Molar Reefs, in the Pompey hard line reefs. These two holes are roughly circular in shape, between 240-295 m in diameter, and 30-40 m deep. They are completely (Cockatoo) or partially (Molar) rimmed by profuse living coral and surrounded by lagoonal depths of 5-10 m. The inner slopes of the Cockatoo blue hole are 60-70° down to a depth of 25 m, below which coalescing sediment fans markedly reduce this angle. At the Molar blue hole, slopes are mainly gentler (45°) and sediment fans and terraces occur below 16 m. Distinct biological/sedimentary associations occur in both holes. Seismic refraction studies across the rim of the blue holes show a shallow (8.5-11 m) pre-Holocene surface beneath the rims. The balance of evidence suggests that the blue holes represent collapsed dolines which may have taken more than one low sea-level period to form. The original surface structures have been modified by subaerial solution processes, and subsequent sediment infill and coral growth following the Holocene transgression.