 Japanese title card · “Secret of the Underground Valley” | |
| Series | Sonic X (ソニックX), TMS Entertainment |
| Season | Season 3 (2005–2006) |
| Episode | Season 3, episode 10 · no. 62 of 78 |
| English title | “An Underground Odyssey” (4Kids) |
| Japanese title | 地下渓谷の秘密 (Chika Keikoku no Himitsu, “Secret of the Underground Valley”) |
| Japan | Not shown on Japanese TV until 8 April 2020 (Kids Station) |
| First aired (US) | 12 November 2005 · 4Kids TV (Fox) |
| Directed by | Mitsutaka Noshitani |
| Written by | Kiyoko Yoshimura |
| Storyboard | Toshihiko Masuda |
| Previous episode | Ship of Doom (S03E09) |
| Next episode | Station Break-In (S03E11) |
Metarex SagaTails & Cosmo
An Underground Odyssey is episode 10 of Season 3 of Sonic X and the 62nd of the anime’s 78 episodes. In Japan it is called “Secret of the Underground Valley” (地下渓谷の秘密). It premiered in English: 4Kids aired it on 4Kids TV (Fox) on 12 November 2005, after a French TV debut on 20 March 2005, while Japanese TV only showed it on 8 April 2020. It was directed by Mitsutaka Noshitani and written by Kiyoko Yoshimura.
Synopsis
Cosmo relives the day the Metarex warship destroyed the ship carrying her people and left her alone. A few hours later an Emerald signal leads Tails, Knuckles and Cosmo across a new planet – Alabasta, in Wikipedia’s account – past a deserted city to a gorge with a cave in its side.
Tails and Cosmo go in, and Cosmo presses the wrong button: an avalanche seals the entrance and knocks Tails out, and she bandages his head. Following the signal deeper, they find a huge Metarex factory and slip inside unseen. At the heart of its production line, which is turning out fake Chaos Emeralds, sits the real one.
They get away with it, but the Metarex Viper hunts them down, Tails is knocked out again and the gem is lost. He gets up to fight for it while Cosmo grabs it back, and Chris, Sonic, Amy, Cream and Cheese arrive in time to finish the job. Along the way, Tails and Cosmo start to realise how much they like each other.
Key Moments
- Tails and Cosmo discover a factory making fake Chaos Emeralds.
- The Metarex Viper hunts them.
- Tails and Cosmo grow closer.
Trivia
- Kiyoko Yoshimura’s script; she wrote 12 of Season 3’s 26 episodes.
- On home video: the The Complete 5th Season DVD set (2006); and Discotek’s Sonic X Collection 2 (2016) and complete-series Blu-rays.
Watch the Episode
The full 4Kids English episode, uploaded by TMS Entertainment’s official “Anime! on TMS” YouTube channel (checked 6 November 2026).
Part of our Sonic X Season 3 guide. See the Sonic X series page for the cast, theme songs and home video.
Sources: titles, credits and air dates from Wikipedia’s list of Sonic X episodes and Sonic Wiki Zone, cross-checked with each other and with Luke’s original Sonic Scene episode table; synopsis written for Sonic Scene from the episode, Wikipedia’s summary and Sonic Wiki Zone’s plot notes; title card and screenshots via Sonic Wiki Zone, copied to our media library.