The Moon Pool is a special, supernatural pool with the power to transform normal people into merpeople when a the full moon passes over the top of the cave.
These places were formed on Earth after collisions with a comet containing moon rocks and crystals that generated special powers corresponding to the moon (revealed in A Magnetic Attraction). The Moon Pool was destroyed in the finale of the third season. It is somehow repaired and ready for the the 26-episode spin of Mako: Island of Secrets
In the early seventeenth century, a mermaid named Eva transformed in a moon pool in an Irish sea cave. During that time, a comet was about to hit the earth and she used her power to form the original Tower of Light to change its course. In the late 1990s, a young girl named Bella was transformed in the same place when she was nine years old. The first known people to become mermaids in Australia were Louise Chatham, Gracie Watsford, and Julia Dove, all of whom were transformed in the 1950's and were given special powers. It is possible that because they were in the Moon Pool at the time of a special planetary alignment with the full moon, their powers were taken away and never given back.
In the mid-2000s, Emma Gilbert, Cleo Sertori, and Rikki Chadwick were transformed into mermaids in the moon pool. Charlotte Watsford, Gracie's granddaughter also became a mermaid in the same place. However, as she fell into the Moon Pool during the most recent Planetary alignment, her powers were taken away forever.
Sometime after the events of Season 3, the Moon Pool is once again active, despite of its destruction, in Mako Mermaids where a young boy named Zac becomes a merman when he falls into it. In Mako Mermaids, the Moon Pool is completely different in appearance than how it appeared in H20; the original entrance from land has disappeared.
The Moon Pool's magic is active only during a full moon. During this time, any human being who jumps into the pool when the moon passes over will be turned into a mermaid or merman, and given special individual powers.
The Moon and the pool generate different magnetism and effects depending on the moon's phases and the planetary alignments.
When nearing collision with a comet, the pool's temperature will heat up in preparation for it.