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. 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.
H2O: Just Add Water
In 2000s, Cleo, Emma and Rikki accidentally go to Mako Island. While looking for a mobile signal, they fall into a cave, where they find the Moon Pool. The Full Moon rises, and aligns with the Moon Pool, releasing the magic in the water. The girls are transformed into mermaids. Days later Emma found a locket at the bottom of the Moon Pool, indicating that they were not the first to be there. Linda Denman, a marine biologist, arrives to Mako, in order to study the fish. She and her team discover the underwater cave that leads to Moon Pool. With the help of underwater cameras, Denman discovers that Cleo, Emma and Rikki are mermaids, and starts to chase them. Louise Chatham, an old woman, reveals that during a lunar eclipse, the Moon Pool will react to the moon magically and the mermaid that is in the pool when the eclipse occurs, will lose her powers. The girls deceive Dr. Denman, who believes that they have lost their powers forever, and leaves the city. After 12 hours the girls again change to mermaids when they come in contact with water.
During planetary alignment, Cleo, Emma and Rikki get moonstruck, and go to the Moon Pool on Full Moon, which results their powers being upgraded. Cleo now controls and manipulates air, Emma can manipulate and control ice and Rikki has full control over fire and electricity.
Charlotte entered the Moon Pool alone, and received all the normal powers and the amplified ones. Charlotte believes she is a "super-mermaid" and takes Cleo's locket, claiming that Cleo does not deserve to be a mermaid. Max says to Lewis that there is a other alignment which happens in every 50 years. If a mermaid is in the moon pool during this alignment, she will lose all her powers forever. Charlotte devises a plan and goes to Mako Island during the full moon despite the girls warning her about it. There she battles with the girls, but loses, and ceases to be a mermaid.
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.
Mako: Island of Secrets
In an alternate storyline, a mermaid pod resides around Mako. Zac Blakely discovers a tunnel entrance on a full moon, which leads him into the Moon Pool when he touches the symbol at the end of the cave. He and the mermaids Lyla, Sirena and Nixie later discover a weapon stored inside: a Trident left behind after a war between mermaids and mermen.
After a brief rivalry, Zac and the mermaids come together to stop a boy named Cam Mitchell from becoming a merman and bringing the Trident into the Moon Pool, which will destroy the Moon Pool.
They later discover a Merman Chamber hidden in a different location on the island.
H2O - Mermaid Adventures