Here is my theory about R2 and early names of levels in Sonic CD.
R1: Salad Plain -> Palmtree Panic
R2: Ridicule Root (that level had only one name before it was merged with R4)
R3: ??? -> Collision Chaos
R4: Dubious Depths -> Tidal Tempest (both names fit the level)
R5: ??? -> Quartz Quadrant
R6: Crazy Toy Box -> Wacky Workbench
R7: ??? -> Stardust Speedway
R8: ??? -> Metallic Madness
When Naoto Ohshima revealed the name of R2, my first thought was that Dubious Depths is an earlier name of Tidal Tempest. The word "depths" is usually used to describe underwater areas.
Later, Masato Nishimura mentioned another name: Ridicule Root. And if R2 never made it past concept stage, why would it have a name change, like other levels?
Something really conspicuous happened with R2 and R4.
We assume that R2 actually had an animated sequence in Sonic CD's ending.
But when I asked Ohshima about that sequence, he replied: "I don't think this cut is in Zone 2."
My first logical explanation was: "Then it has to be Tidal Tempest, because all other Zones are already represented." Well, the truth is probably somewhere in between.
We know that R2 does not appear in the game, and R4 does not appear in the animation. Or does it? I compared colours, machinery, plants in Tidal Tempest to what is seen in that segment of the ending. It looks similar.
But where is water, and why R2's Antlion appears in that clip? My theory is that it represents neither R2, nor R4. That's why it was not featured in the original Mega-CD version of Sonic CD.
There are two underground Rounds. Main differences: 2R is much bigger than 4R, the latter has a lot of water. Basically, that's it. Those areas are rather sketchy. Developers probably wanted locations with more room for Sonic to run around. Judging by experience from Sonic The Hedgehog (1991), they could see that Marble Zone and Labyrinth Zone are not very suitable places to include in a fast-paced platformer. So they had to decide which of two Rounds should appear in the final game. From the beginning of 1992 (see the map)...
...to the end of that year the decision was not made. At some point, they had to deliver concept art to Toei Animation, and the studio would make animated opening and ending for Sonic CD. The latter would include all Rounds from the game, just like credits in Sonic 1 showed all Zones from that game.
It seems that development team could not decide which level they would use even after they sent concept art to Toei. So the studio created a sequence based on some artwork, completely unknown to us. It's not like rainbow waterfall, that is associated with Round 2. But it features roots, just like in the name "Ridicule Root."
The fact that this clip appeared in later editions of Sonic CD can mean 3 things:
1. It was included for "completion" reasons. And if the full animated ending is now complete...
2. ...Toei never created a clip with a water level. Why?
a). developers wanted to use only one underground Round. Water would only slow Sonic down, so at one stage, Ridicule Root was in priority.
b). animation specialists were instructed very vaguely about two locations that can look very similar, so one can replace the other.
3. Toei created clips for all 8 Rounds, but for some reason Tidal Tempest one was lost, so SEGA opted for using Ridicule Root clip as a bonus, even if that level was not featured in any edition of Sonic CD.
But that's only guesses. What we actually have is Sonic CD with Ridicule Root in the credits and Tidal Tempest in the game itself. Strange, isn't it?
For the most part of 1992, CD Sonic was supposed to be a game with 8 levels, CD-quality "Past" mixes... It looks like that by 1993 developers realized there would not be enough space to include all this stuff. So they had to make the hardware handle "Past" music and use only one "slow" stage. They could have excluded Wacky Workbench from the list, right? Well, the logic was different, I suppose. Yes, Wacky Workbench is not a "fast" stage, but it has a gimmick that's unique. Sonic 1 did not have anything like that. And when the team finally decided to use Tidal Tempest instead of Ridicule Root, "uniqueness" came to the front. Because it's the only place on Little Planet where Sonic can explore underwater depths.
My another theory is that there are different development documents that cover what was left out of the final product. Naoto Ohshima could have found information about a dungeon that never made it into Sonic CD and "Dubious Depths" which could have been an earlier name of Tidal Tempest – and that Round was not used, too (in animation, that is). Masato Nishimura could have looked up "Ridicule Root", probably the only name of R2. It would make sense if R4 was called Dubious Depths at one point of development. Right at the start the only way is down to the depths of uncertainty.
Developers would change the names of the levels which made it to the final list. Even earlier mentioned Wacky Workbench was called Crazy Toy Box earlier. Why they would change "Ridicule Root" to anything else, if they would not develop R2 further? They could have decided to move elements of R2 to other places.
Fans would appear in Tidal Tempest, just like the whole "underground" theme would be further explored in that level (especially in "Past" version). Antlion was planned to appear in R8: Metallic Madness.
While at Toei, the robot was featured in Ridicule Root clip. And rainbow water – in the intro.
Music of R2 would be used in "D.A. Garden" mode and remixed on Sonic The Hedgehog - Remix album. Soundtrack to Ridicule Root has a forest/jungle vibe to it, just like the name. Judging by the map, surface level played a role in the whole concept, what with trees, swinging platforms and stuff. That's why Dubious Depths doesn't fit as the final name of R2 – it only describes, well, the depths. R4 has a very short surface part at the start, that's all. That's why Dubious Depths would fit R4, maybe more than Tidal Tempest.
Concept art for R2 continues a "tradition" of Marble Zone: using ancient architecture (this time it's Roman aqueduct instead of Greek structures), mountains in the background... and no sign of any underground areas.
Marble was mostly about that dungeon with lava and spikes, but it had places above, too. I suppose Ridicule Root was something like that. Aqueducts above, traps and roots below.
Let's look at boss of R2.
The main motif here is a big spinning wheel. How do you make an underwater version of it? You replace tires with bubbles!
Also note that R2 boss art is rather sketchy, with no background, compared to R4 boss art.
In conclusion I would like to say that the only way to know for sure is to hear what developers of Sonic CD say and read what they write. Information does not appear out of nowhere. People share it. We could have waited years, decades maybe to find out the name of R2. And it would go like: Ohshima / Nishimura / Hoshino decided to share a bit of trivia and posted on social media something about development of Sonic CD. "I've been cleaning up my house and found a design document. Here is a fun fact: Stardust Speedway was originally known as ... " – a developer would write in 2034. Or someone could just ask him/her in 2024. Anyway, that's what I did last year and got an answer that I could have had if I just thought about writing to Ohshima much earlier (say, in 2015). I had an honor to find out stuff and it's not an indication of how cool I am, but actually how cool Ohshima is. He found time in his busy schedule to check out information from 30+ years ago and share it. With a fan of Sonic. It could have been you, who are reading this right now. I am not encouraging to start sending hundreds of messages from everyone in Sonic community to developers of our favorite games. Let's just come up with a question and ask them politely. Like: "Could you tell please, why Tidal Tempest was not featured in ending sequence of Sonic CD?" You can ask that, I can ask – but let's not do it together and at the same time. We are all people, and it can be irritating to get the same question from everyone. It's better to think what other puzzles still remain unsolved in history of Sonic and what we can ask to get closer to solution. And if a person doesn't remember / doesn't want to answer / doesn't have time – the best thing to do is to leave that person alone.
