"An Underground Odyssey" (地下渓谷の秘密 Chika Keikoku no Himitsu?, lit. "Secret of the Underground Valley") is the sixty-second episode of the anime series, Sonic X. It first aired on 20 March 2005 and 12 November 2005 in France and United States respectively.
The show opens with a sequence inside Cosmo's ship and ends when Scarship (see previous episode) destroys it, leaving Cosmo alone. Then, the Blue Typhoon lands on a new planet. It seems that there is a Chaos Emerald nearby. Tails, Knuckles and Cosmo follow the emerald's track to an abandoned city. Knuckles discovers a deep gorge with a cave. Tails and Cosmo enter, but Cosmo accidentally pushes a button which launches an avalanche and seals them inside the cave. Tails is knocked unconscious, but Cosmo wakes him up and he dizzily sits up and holds his head where he got hit during the avalanche and Cosmo puts a bandage on his head.
Tails still receives the emerald's signal and he and Cosmo pursue their searches. Inside the Blue Typhoon, Amy receives a distress call from Knuckles and she informs Chris, Sonic, Cream and Cheese (who are all aboard the X-Tornado) of the situation.
Inside the cave, Tails and Cosmo discover a big factory full of Metarex. They enter unnoticed and they find the Chaos Emerald inside a manufacturing plant of fake emeralds. They are found out by the Metarex, but they are able to escape with the real emerald. Then, Tails fools the Metarex with special "fireflies". Regardless, Tails and Cosmo are found and retrieved by the Metarex and Tails gets knocked out and drops the Chaos Emerald. The Metarex steal it. However, Cosmo manages to wake Tails up and he yells to give the Emerald back. The Metarex tells them it's impossible and Tails fights it. The Chaos Emerald drops, Cosmo retrieves it, Tails is caught by a Metarex. Fortunately for them Sonic, Chris, Amy, Cream and Cheese arrives to help them together they defeat the Metarex and they are able to escape with the Chaos Emerald. Tails and Cosmo also are starting to have feelings for each other.