Mako Island was formed after a comet crashed into the Earth. We know that one of the shards impacted in Ireland forming the Sea Caves of Ireland. So other shards may have landed in other locations creating moon pools wherever they impacted. It is possible that these other moon pools could have also created mermaids all over the Earth in other places and other times. Its origin is revealed in Season 3. Around Mako there are the largest population of Mako sharks and sea turtles, some of which cannot be found anywhere else on earth.