Rockall

The small rocky island of Rockall, with a Irish naval patrol boat.

Navigatio Brandani 26.8–9: 

Erat autem parva et nimis rotunda illa īnsula, quasi ūnīus stadiī. (9) Dē terrā vērō nihil habuit dēsuper, sed petra nūda in modum silicis appāruit.  Quantum erat lātitūdinis et longitūdinis, tantae altitūdinis.

The uninhabited island of Rockall, claimed by the United Kingdom, sits alone in the North Atlantic.  It is as almost high as it is wide, 71 feet high and 90 feet at its widest, so about 280 feet in circumference, whereas a Roman stade was 625 feet.