The Samaria Gorge trail starts at the top of the White Mountains and descends more than 8000' to the sea. That is a descent that is larger than the hike to the bottom of the Grand Canyon. Unlike the Grand Canyon, however, you don't have to hike back out. You can hop a ferry in the town and take it back to the road down the coast. The canyon is cut into limestones, as seen in this image.