Underground is the fifth stage from Streets of Rage 4.
- This stage contains lots of visual references to other franchises, which can easily be spotted on the backgrounds of the stage:
- Right in the first screen of the level, there is a small set of drawings, which contains four turtles with different colors (red, orange, blue and purple) and a rat looking at them from the right. They represent, respectively: Raphael, Michelangelo, Leonardo , Donatello and Splinter. This is a clear reference to the TMNT franchise, which is also known for its beat-em-up games, loved by many.
- Some of the graffiti on the large pipes have the names of some of the enemies, such as "Big Ben" or "Koobo".
- There's a large graffiti of a skull with octopus tentacles coming from it, which resembles Marvel's HYDRA logo.
- Right after the second small bridge, there's a wall painting of Sonic doing his signature Spin Dash technique on top of what looks like the classic Green Hill Zone's ground next to some rings. On the bathroom area of the level, there are two doodles of Sonic, one of them on a pipe near the ceiling, and the other on the corner of a wall.
- There's a large horizontal graffiti of Joe Musashi's head from the Shinobi series, in what looks like his Shinobi III outfit.
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