Mako Island was formed after a comet crashed into the Earth. We know that one of the shards impacted near Ireland (Eire) 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. It's origin is revealed in season 3. Mako Island has a magical sea cave at its base leading to the Moon Pool, where Cleo, Emma, and Rikki, and the former mermaids, Gracie, Louise, Julia, and Charlotte, got their mermaid powers. There is also a secondary cave leading to a waterfall outside. The waterfall is part of a stream running through a mangrove forest. Mako Island has been around long before humans walked the earth and it will still be around long after humans have gone. Around Mako there are the largest population of Mako sharks and sea turtles, some of which cannot be found anywhere else on earth.
Off-shore there are coral reef sands and a number of wrecks, including Miss Louise Chatham's boat the Lorelei. On the northern side of the island there is a place called Triton's Reef, its a known as place where sharks breed. Throughout history, there have been peculiar happenings around Mako, such as a whirlpool and numerous ship wrecks and disappearances, giving Mako an eerie reputation.
When the Comet Eva returned to Earth in 2010, the whole island heated up like it was working as a normal volcano. That lead to very strange temperature rising, and the fish were fleeing from the vicinity of Mako Island. The comet entered the atmosphere on February 7th which was the same day when the water first attacked Cleo, Rikki, and Bella.