The Oracle of Delphius could actually be the true Big Bad of the show. He has amazing powers and could have killed Robotnik in his own bedroom as shown in a flashback in the first episode, but he instead gives him the heads up about the prophecy, which gets Robotnik to start looking for the trio. Also in the same episode, if he hadn't directed the trio to meet at a public place then their foster parents would have been safe and not turned into robots; as if the Oracle is going with an "I'll finish off whoever is left in the end" approach. Alternatively, the Oracle just likes to mess around with people. Alternatively, he doesn't do any of those things because averting from the prophecy would make it not come true. Or he's doing this to prevent something far worse from happening. whatever that may be.
There's a point where Sleet and Dingo stop being typical Saturday morning cartoon villains and become tired employees threatened into a job they don't want at all, even purposely engaging in counterproductive work behavior to spite their boss.
Is the prophecy Delphius gave to Queen Aleena even true?
Audience-Alienating Premise: There are many Sonic fans who couldn't get into it even back when it was new, let alone in the modern age; on one hand, some fans were still bitter that there wasn't a season 3 of SatAM, and this was on top of it being different from the games even at the time. On the other hand, the show came out shortly before Sonic Adventure, itself different from Underground, changed the direction of the franchise for years to come, and even today still has more influence than this show did. Basically, the show is an oddity no matter how you look at it, since it doesn't exactly gel well with what came before or after it.
Base-Breaking Character: Bartleby, Sonia's fiancé, is either hated for being a cowardly snob, or liked for being a morally grey neutral party in a black-and-white conflict with decent amounts of Character Development.
In "Tangled Webs", Sonic eats a flower for no reason at all.
During the episode song of "Winner Fakes All", Sonic briefly runs past and nearly accidentally flips up the skirt of a human girl, one of the few humans other than Robotnik, and the only one that isn't just background filler, which is given no explanation whatsoever.
Complete Monster: Dr. Robotnik is as power-hungry and evil as he has ever been. Having invaded and conquered Mobius years before through sheer force and Unwilling Roboticization of a vast majority of its population, Robotnik now rules with an iron fist as he seeks to round up any remaining organic beings to have them enslaved as his cyborgs. Along with regularly abusing and threatening roboticization or death to his Co-Dragons Sleet and Dingo, Robotnik frequently betrays his own allies to serve his purposes. Not restricted solely to roboticizing those who stand against him, Robotnik hopes to find the Sanctuary where resistance fighters raise their children so he may turn it into a "cemetery", and on more than one occasion Robotnik tries to wipe out entire cities of life to demonstrate his might. Robotnik puts Sonic and his siblings through a variety of life-threatening and sadistic choices in his quest to find and destroy their mother, and when all of Mobius is threatened by a Chaos Emerald storm, Robotnik is willing to let the world be wiped out just to spite Sonic.
Crossover Ship: There's a small subset of fans that ship Sonia with Shadow the Hedgehog or Espio the Chameleon from the main continuity. Alongside that, Manic with Scourge the Hedgehog from Archie Sonic has its fans too. Sonia is additionally paired up with Silver the Hedgehog as a Battle Couple due to the popular romhack/fangame S-Factor that features the two of them as the playable characters (with Scourge along for the ride.)
Cult Classic: While the show didn’t really do that great in reviews or ratings, it does have a small, but loyal fan following.
Mindy LaTour only appeared once, but she's a popular character for being childhood friends with Sonia, having an interesting character design, and being the only non-band member to sing lead vocals.
Sonia, Manic, and to a lesser extent Aleena are this for the entire franchise, as many fans like them and use them in fan works, even if they are not familiar with or even fans of Sonic Underground as a whole.
Epileptic Trees: Thanks to so many original characters and the omission of almost all the official characters existing at the time, despite there being potential room for them, and since many of the background characters were more alien than any Earth-based animal, many Sonic fans and even fans of the series debated whether it was really even intended to be a Sonic show, and that the characters of Sonic and Dr. Robotnik were just a hook for Sonic fans. Even the series plot is basically a This Is Your Premise on Drugs take of the Sonic vs. Robotnik story.
Evil Is Cool: This would be a primary reason for why Special Agent Sleet has as many fans as he does.
Fanfic Fuel: Who was the royal triplet's father? Many fans speculate through fanfiction that Sonic, Sonia and Manic are twins/half-siblings and have two different fathers, due to Sonia most resembling her mother Queen Aleena and Sonic and Manic most resembling either his father.
Fan Nickname: The overpowered, Chaos Emerald-fused version of Dingo has sometimes been called "Chaos Dingo".
Fanon: The show never explored the trio's Disappeared Dad. Fan Fiction and Fan Art, however, speculate that his name might have been Jules (essentially emigrating the Archie Comics character into this universe). His purported appearance, however, varies — some say his fur color was blue like his son Sonic's. Others say it was green like Manic's.
In "Art of Destruction", Manic complains to his siblings and the recently reformed ART that the prophecy never said when they would win, and that it could be twenty years later. Given the show was Cut Short before they could win, and the Development Hell and eventual cancellation of the epilogue meant to be in Sonic Universe, it's depressing if you were a fan of the show.
"Virtual Danger", the last aired episode of the show, ends with ships being flown into the tower that is Robotnik's main base, causing it to explode. This aired about two years before the 9/11 attacks.
When the halves of the names Sonia and Manic are switched, they make up the words "Sonic Mania".
Chomps, Knuckles' pet dinosaur, shares almost the exact same colors as the Biolizard and has a near-identical body-shape.
One of the early concepts for Sonic 1 when it was still in production was to have Sonic as part of a rock band, though sources say they had to take that part of the game out to save space on the cart, and that Sega of America told their Japanese counterpart that that concept for Sonic wouldn't go over well with American audiences.
One episode has Sonic playing tennis against himself, and throwing a John McEnroe-esque tantrum when the umpire (also himself) makes a call against him. Twenty years later, the second trailer for the film adaptation has Sonic playing both table tennis and baseball against himself.
In "No Hedgehog is an Island", when Dingo double-crosses Sleet and plans to become ruler of Mobius himself, Sleet objects on the basis that "[He] can't even drink a glass of water without using both hands!". Twenty-one years later, Donald Trump drinking a glass of water with two hands was seized upon by opponents for jokes and as a sign of him being physically unfit for office.
Manic is a thief and a high-tech specialist who rides a Hover Board, all of which anticipates the Babylon Rogues of the Sonic Riders series. Bonus points for being green just like Jet the Hawk.
At the end of the second episode, Sonia kisses Sonic on the cheek for a job well done, and Sonic turns to Manic and says, "You're not gonna kiss me, are you?" Manic's answer? "No way, but you are kinda cute." This is one of the funniest lines on the show.
In another episode, Sonic asks for a dance while looking Sonia's direction. She accepts, but then he says he was talking to Manic, and the two begin slow dancing. The pair had a surprising amount of this.
The theme song has been synced to numerous webcomics and bottom-text memes, notably Ctrl+Alt+Del.
I am Hothep, you are hedgehogs, we know not each other, but Aman-Rapi smile on me if I help find queen mother!Explanation A small part of the lyrics of "Mummy Wrap" from the episode "Mummy Dearest". It became a meme after a level in Sunky.MPEG featured the part where Hothep says the lyrics as its background, except that it's looped ad nauseam. This also helped turning Hothep into somewhat of an Ensemble Dark Horse.
Robotnik firmly crossed this in "Three Hedgehogs and a Baby" when he tricked Manic by letting him find Hip, a surveillance robot that resembled a living baby hedgehog, in the hopes that he'd get attached to it and accordingly take it to — and thus reveal the location of — Sanctuary, a hidden place of refuge for children and infants. So he could destroy it, then roboticize and enslave everyone there:
Robotnik: When we find it, Sanctuary will become... Cemetery!
Jaleel White voices Sonic, Manic and Sonia. As one might expect, he does fine with the first two, but Sonia ends up sounding like a cross-dressing Steve Urkel, which neuters any scene she has that's meant to be taken seriously.
While Sleet is shown to be a reasonably competent villain, his Peter Lorre-esque voice — and Ren-esque tendency to call Dingo an "EEEEEEDIOT" — makes it hard to take him seriously a lot of the time.
The characters' constant smiling in the early episodes.
The death scene of Hip, the exploding baby hedgehog robot. All of it.
By the time the series premiered, fans were used to Jaleel White's voice as Sonic. The voice he used for Manic isn't too bad, either, and sounds distinct from his brother's. But having White voice Sonia is really pushing it and a prime source of Narm. She sounds an awful lot like Sonic pretending to do a woman's voice and failing. Most agree that her singing voice, Stevie Vallance, sounds much better in the role and should have fully voiced her.
Knuckles' voice sounds like a nasal, slightly higher version of Manic's voice, even though a deeper tone would better suit the character. And this from Brian Drummond, known for Ocean's Vegeta — and much later, Dr. Eggman and his various incarnations — of all characters.
While Maurice LaMarche was a good casting choice for Sleet, the decision to have him sound like Peter Lorre doesn't match Sleet’s character at all. Tellingly, many dubs, such as the German dub, give Sleet a deeper and more respectable voice.
Signature Scene: The scene from "Three Hedgehogs and a Baby" where Manic fails to save the titular baby and it explodes. It's either Narm or horrifying depending on who you ask, but it's hard to forget a robot baby dying.
So Bad, It's Good: With the show's In Name Only relationship to Sonic, the poor animation quality and the plot, the show is considered as this by some fans.
The series received one in SpacePOP, a girl-targeted show about royalty going underground as a rock band to search for their missing parents against a dictator with Mecha-Mooks and a hired bounty hunter.
Thanks to the reuse of Mobius, roboticizing, and other aesthetics, Underground itself was likely intended to be one to Sonic the Hedgehog (SatAM).
Strawman Has a Point: In "Art of Destruction", Manic complains that no matter how far they go or what they do, Robotnik will just throw something else at them, and the situation doesn't ever seem to change no matter what they do. When you look at many of the previous episodes where the robots seemed to only escalate in how powerful they are, as well as the generally unchanging nature of the show, it's hard to not agree with Manic.
"Winner Fakes All" has Robotnik trying to build a robot that can outrun Sonic in order to lead the heroes into a trap. This would've been the perfect way to introduce Metal Sonic, especially with said plot working well to set him up as a tougher, deadlier foe than anyone they've faced before. Sadly not; instead, it's just a generic, turtle-looking 'bot that serves no purpose to the overall plot and is defeated comfortably.
"Bug!" is about a swarm of robotic flies with the ability to mind-control people with a serum, which ends up affecting Manic. This could've been a great opportunity to have the bond of the three siblings grow stronger as they try to diagnose and cure Manic's brainwashing, but they quickly find out what's wrong and fix it, then Robotnik gives up as soon as he loses connection with Manic.
Some of the characters in this show really come close to this territory. For example, look at Sonia's adoptive mother in the first episode; she looks like someone pulled out of a Renaissance Fair.
This version of Mobius has plenty of insectoids and bugs, with some looking monster-ish.
The Do-Bot in "Haircraft in Space."
Dingo's character model sheets depict him with a jarring amount of lines◊ in his face compared to the rest of his body. Thankfully, this was toned down in the cartoon itself.
The robot baby from "Three Hedgehogs and a Baby" is infamous for this because it doesn't have the same eye pattern as most hedgehogs in the series, but has smaller, ape-like eyes. Many people have compared it to Sonic's original live-action movie design.
for the film adaptation has Sonic playing both table tennis and baseball against himself.
