The "Seven Wonders of the World" were: The Great Pyramid of Giza; The Hanging Gardens of Babylon; The Statue of Zeus at Olympia; The Temple of Artemis at Ephesus; The Mausoleum at Halicarnassus; The Colossus of Rhodes; The Lighthouse of Alexandria. You can find out more at Alaa Ashmawy's site here.
Natural wonders have fascinated writers since Herodotus' Historia (5th Century BCE). Solinus' De mirabilibus mundi ("Wonders of the world" : biography) was well known in the medieval period*. Nennius' represent an early British example of what was to become a significant trend in medieval writing: delight in the odd. For a crowning example of this trend, see Sir John Mandeville's (Biography/Discussion) Travels (~1372 CE).