The Shape of Time (Rymworld Arcana Book One)

Locked away in a secret prison in the middle of the Bermuda Triangle, fourteen-year-olds Antares and Magdavellâia set out to discover a lost and forgotten island that holds the key to foiling the evil mystics' plan to control time. Following a series of intricately concealed clues left behind by Magdavellía's parents (clues Antares seems to be the key to unlocking), they must wield both geometry and alchemy, outsmart molemen and mermaids, and outrun fiendish outer space aliens--all while attempting to solve a riddle as old and mystifying as the sun.