H2 Oh No (すいちゅう ひみつきち lit. Underwater Secret Base in Japanese) is the tenth level and is the last level of the second warp room in Crash Bandicoot: The Wrath of Cortex. The first part of the level takes place underwater, where Crash maneuvers a submarine (like in the second part of Sea Shell Shenanigans and the first part of Coral Canyon), shooting enemies and breaking crates. The second part of the level takes place in a damp cave, where Crash traverses on foot, encountering lab assistants and crossing pits of green electrical fluid on iron crates.
Its name is a portmanteau of H2O, the chemical formula for water (2 hydrogen atoms and 1 oxygen atom) and the phrase "Oh no".
The water was going to change color in the beta version. However, there are hidden parts in the underwater segment where the player can see a bit of it from the corner of the screen. Some can be found in the Bonus Round by doing a slide jump, then jumping again and using the Death Tornado Spin to glide further. When Crash falls into the water, the camera will briefly bob downwards to reveal the color changing water beneath the normal one.
Early screenshots show how the purple jellyfish, encountered in the underwater section, were also going to appear in the platforming portion of the level. One possible reason why they were removed is because of drawing problems, as it seemed that a small part of their bodies goes through the floor in a distance. Since this little error was never fixed, they were scrapped.