The largest gathering of the largest fish on the planet--over four hundred whale sharks--assembles in the same small patch of ocean just off Mexico’s Yucatan Peninsula. Why are they here? Mysterious Planet embarks on an epic journey to find out: from the clear blue waters of the Caribbean, into the largest network of flooded caves on Earth, spiritual underworld of a lost civilization, home to monsters and alien life forms ,where a meteorite the size of Manhattan is hurtling towards planet Earth, on course to kickstart the evolution of a whole new line of ocean giants.