|“||[A]ll shadows are part of the same substance. There is only one darkness, and creatures of the Underworld can use it as a road, or a door.||”|
–Nico di Angelo explains shadow travel to Percy Jackson, The Last Olympian
Shadow travel is a form of transportation that allows creatures of the Underworld and demigod children of Underworld deities to travel to any desired place on Earth or in the Underworld, although it makes the user extremely fatigued.
Hellhounds and children of Hades use this traveling method by slipping into the shadows and coming out somewhere else of their desire. Children of Pluto can also shadow travel, as shown by Hazel Levesque in the House of Hades.
Exactly what happens in the shadows is not known. It is to be assumed that it does not simply take them there, but acts as a tunnel, as when Mrs. O'Leary shadow traveled she was still running after they got into the shadows when Percy Jackson and Nico di Angelo were traveling to Westport, Connecticut to find May Castellan. Percy describes how it feels to shadow travel as "going so fast that it feels like your face is peeling off."
Shadow traveling exhausts the user greatly; for example, Nico said that he passed out for a week in China when he first tried it, and Mrs. O'Leary herself was tired after doing it. The more you do it, the more you lose contact with the physical world.
Shadow traveling was first shown by Mrs O'Leary, who took Nico and Percy to May Castellan, Luke's mom, at Nico's request to figure out Luke's trail and memories (and to better help Percy fight Luke since it was realized that Luke had taken a bath in the River Styx). Later, the hellhounds from Kronos' army shadow traveled inside the defenders' borders to attack during the Battle of Manhattan. Nico is also able to shadow travel, but it requires a lot of effort and it leaves him exhausted.
After an attack from the mountain gods, Nico mentions that he could shadow travel past the mountains if he were on his own and his strength was up.
When they reach the House of Hades, they are separated. Frank calls in a favor from the dead to transport the Seven to where Hazel is battling Clytius. It is unknown what type of travel this is, shadow or other. After the battle with Clytius, the ceiling starts to break and crumble. Hazel asks Nico whether he could shadow-transport them, but Nico says he can hardly transport himself. Then, Hazel offers to help and they both successfully transport the entire team to a hillside overlooking the River Acheron. Nico then offers Reyna his help to transport the Athena Parthenos to Camp Half-Blood via shadow travel.
Nico shadow travels himself, Reyna, Coach Hedge, and the Athena Parthenos to Camp Half-Blood to prevent a civil war. Along the way they have an incident in Albania, fight earthen shells in Pompeii, fight lycanthropes in Portugal, go on a cruise ship, fight Orion in Puerto Rico, and fight zombies in South Carolina.
In The Blood of Olympus, one side effect was shown besides exhaustion and fatigue: overexertion of shadow travel can cause the user's molecular structure to begin to fall apart, gradually turning them into shadows and losing themselves. Due to Nico pushing himself to bring the Athena Parthenos to Camp Half-Blood, he began to lose himself, even entering a "shadow coma".
- Nico di Angelo
- Hazel Levesque
- Other unknown children of Hades/Pluto
- Demigod/legacy children of Underworld deities
- In other literature, shadow travel is sometimes referred to as Umbraportation.
- Shadow travel is an ability that seems to be shared by most, if not all, Underworld monsters and deities, meaning gods like Thanatos and Melinoe may also have this ability, but haven't used it in the series.
- According to Hylla Ramírez-Arellano, Shadow Travel is magic.