Nightmare Fuel / The Mysterious Cities of Gold

• The Jade Mask. Dear God, The Jade Mask....
• The documentary of a chicken's head being cut off in a children's show.
• The BBC never screened the documentaries (which also contained re-enactments of sacrifice and grave-robbing), possibly for time constraints as much as content.
• The slaughter of the helpless Mayas by the Olmecs with the Death Ray built into their flying machine.
• The "Great Treasure" itself. It is in all but name a nuclear reactor, but what makes it so terrifying is that despite it's size (small enough that an able-bodied adult can haul it around) it delivers an insane amount of power when active; so much that without it's control key (which is revealed to be Tao's mysterious jar) it goes haywire and begins melting it's way to Earth's core. High Priest explains that once it reaches the core, it would wreak havoc upon all life on the planet if not outright destroy it. It is a testament to how advanced Hivan technology was, but also how frighteningly destructive it could be if incorrectly handled, even moreso than the technology we have today.
• The Family-Unfriendly Death of the High Priest of the City of Gold who is exposed to heat so intense that his clothes literally burst into flame on his body. The fact that he's Esteban's father only makes it worse.
• When the kids are captured by Olmecs. Not only is the first meeting cast very much like an alien abduction, but when they escape the first time, Esteban is essentially electrocuted into unconsciousness, and the Olmecs decide to cryogenically freeze the kids in what look like glass coffins to prevent any escape attempts or injuries. Only the swift thinking of Mendoza gets the kids out of there safely.
• Early on in the first season, Gomez threatens to feed Esteban and Tao to what are strongly implied to be Army Ants in order to get Zia to tell him the message in the Golden Quipu, even going so far as to tell the kids that the ants can strip a cow to the bones in minutes.Of course, it's tempered by the fact that the ants, based on their behavior and appearance, are actually Leafcutter Ants. Still unpleasant, but not nearly as dangerous. Not that Gomez knows that...
• In Season 3, the Golden Condor is knocked into the ocean. When attempting to retrieve it, Zia becomes trapped when clouds prevent the Condor from receiving power. Then the sharks arrive...