I am thinking this may be a prequel to H2O. Not to far before it, maybe a couple years.

If a quake happened near Mako, it could have opened a land entrance, i.e. how Emma, Cleo, and Rikki found the moon pool.

Perhaps Zac opened a land entrance with the trident. We know he ends up with the trident later in the series.

Also, how cool would it be to have a grotto with a moon pool in your house that exits to your pool outside?