Oval reef situated about 900m west of the Gwynedd mainland near Llandwrog. Arianrhod is a figure in Welsh mythology, described in the fourth branch of the Mabinogi, the daughter of Don. Caer Arianrhod is said to be her palace and is associated with the off-shore reef of the same name. RCAHMW aerial reconnaissance over the reef with clear-water conditions and a low tide on 16th June 2010 showed the reef to be oval or pear-shaped, approximately 450m east-west by 215m north-south, rising to a low summit at the west end where it is briefly exposed by the lowest spring tides. It is generally connected to the land by a submerged area of stony ground. No convincing traces of artificial features can be seen in the shallow water around the reef, although there are suggestions of some regular wall lines and a 'D' shaped structure closer to the shoreline, possibility associated with coastal fishing industries.

T. Driver, RCAHMW, 18th June 2010.