Shadow travel

No." Nico scowled. "Pegasi don't like me, and the feeling is mutual. But there's no need for flying." He whistled, and Mrs. O'Leary came loping out of the woods.
"Your friend here can help." Nico patted her head. "You haven't tried shadow travel yet?"
"Shadow travel?"

I was a little nervous, but I leaned down to Mrs. O'Leary's ear. "Okay, girl. Uh, can you take me to Westport, Connecticut? May Castellan's place?"
Mrs. O'Leary sniffed the air. She looked into the gloom of the forest. Then she bounded forward, straight into an oak tree.
Just before we hit, we passed into shadows as cold as the dark side of the moon.

I don't recommend shadow travel if you're scared of:
a) The dark
b) Cold shivers up your spine
c) Strange noises
d) Going so fast you feel like your face is peeling off
In other words, I thought it was awesome. One minute I couldn't see anything. I could only feel Mrs. O'Leary's fur and my fingers wrapped around the bronze links of her dog collar.
The next minute the shadows melted into a new scene. We were on a cliff in the woods of Connecticut.

TLO ch.5-6

Then, from somewhere above us, a voice yelled, ‘Hey, Bronze Butt!’
Over the Colossus’s head, a cloud of darkness formed like a cartoon dialogue bubble. Out of the shadows dropped a furry black monster dog - a hellhound - and astride his back was a young man with a glowing bronze sword.
The weekend was here. Percy Jackson had arrived.

Percy sliced off another one. ‘Hey, Ugly!’ he called down. ‘You don’t need all these pointy things, do you? I’m going to take one to the beach. Mrs O’Leary, fetch!’
Percy tossed the spike like a javelin.
The hellhound barked excitedly. She leaped off the Colossus’s head, vaporized into shadow and reappeared on the ground, bounding after her new bronze stick.

THO ch.36-37