The samples confirmed that it was indeed coral and radiocarbon dating confirmed its age.
Other similar ancient reefs – called relict reefs – have been discovered before, but none as far south as this.
The team think that this reef died when it was flooded as a result of sea levels rising about 7,000 years ago, but the modern temperature at these latitudes also limits coral growth, which is why the relict reef is so much bigger than the modern reef.
Now that sea temperatures are rising, however, reefs may start to grow bigger at higher latitudes.
The relict reef doesn’t have an extensive modern reef attached to it but it does have some individual corals which are newer – from the last 2,000 years.