How is it possible that so many megalithic tombs can be found on a small island in the middle of an estuary? Geology has the answer: some 3,000 years ago the sea level was some 5 to 7 metres below the current level and the coastline hundreds of metres longer, so that very probably Guidoiro Areoso island formed part of a peninsula leading to Arousa island. This explains how the materials to build these megalithic monuments were brought there.