Theory of Everything 2

Theory of Everything 2 (often referred to as ToE2) is the eighteenth and currently the last official level of Geometry Dash, and it is unlocked by collecting 30 secret coins. It is the second level to be rated "Demon" as of Update 1.9. This level is commonly recognized as one of the hardest official levels in normal mode, the other one being Clubstep.

Theory of Everything 2 is considered to be one of the hardest levels among the main 18 along with Clubstep. Which one of the two is harder is a subject of debate, and discussions on several sources have just argued back and forth. It is a matter of personal opinion.

Description
This level introduces Wave sequences at double speed as well as a mini Wave sequence at slow speed. It's the second level (after Blast Processing) to introduce the Wave feature and the first level to introduce the mini Wave. The level also introduces invisible sawblades and slopes. This level does not contain dual or mirror portals.

Secret Coins

 * The first secret coin is located at the first mini ship sequence. After you go through an inverted-gravity portal, immediately hold then let go. You will go down through a small gap to get the coin then get back up to the normal-gravity portal. If you go down too low, you will hit a spike or a block.
 * The second secret coin is located at the second wave sequence. After you maneuver through the third diagonal hallway, try to make your wave form go down through a small entrance to get the secret coin. It is one of the hardest coins to get, as it also requires precision and control to complete the Wave after collecting the coin, as the wave is at double speed and the opening is extremely tiny for the Wave to enter.
 * The third secret coin is hidden above the course. After you exit the reversed gravity ship section (the one after the slowed cube sequence), jump on the two jump rings, even the one marked with a cross. If you do this, you will land on a platform. Then perform a late jump, collect the coin, and jump again on the edge of the platform to exit safely before you enter the last UFO sequence. This sounds easy, but it is hard without practice, since the timing is critical. If you do not jump at the edge, you will land before the platform and crash. Since this part is fast-paced, it is also quite difficult to jump off the edge of a platform. If you are too late, the cube will just fall off the platform without making the jump and you will also crash.

Trivia

 * In Theory of Everything, all of the portals that existed when the course was released could be found in the level. However, in Theory of Everything 2, there are not any mirror or dual portals (mirror portals were last used in Electroman Adventures and dual portals were last used in Blast Processing).
 * In the half-pipe at the end, a monster that resembles the monsters in the Update 2.0 preview can be seen with an eye (blue ring) and an open mouth with fake spikes to slide through.
 * The icon unlocked for completing Theory of Everything 2 also resembles the Update 2.0 monsters, and it resembles the Blast Processing icon. The only difference is that the ToE 2 icon has more "teeth", and they are arranged in a different pattern.
 * Completing the level takes about 1:32. However, if you use the first UFO speed glitch (refer to errors), it will take around 1:30 to complete.
 * This level and Hexagon Force are the only official levels to include a slow speed portal and the normal speed portal (first seen in Update 1.8, despite the slow and normal speed portals being introduced in Electrodynamix in Update 1.7)
 * This level has both the longest and shortest slow portal sections.
 * This level is the only level to feature the slow portal for UFO, cube, and mini-wave as to date and is the only level to have multiple slow portals.
 * This is the only level to have 2 composers (DJ-Nate and F-777) instead of one.

Errors

 * During the slow ship segment, at about 39%, there is a design error where one of the shaded blocks is slightly pushed to the left.
 * There is an error in the first mini ship section of this level. You can skip the third gravity portal and the slow speed portal by going under them. If you skip the speed portal, however, crucial timing will be needed to pass the UFO sequence that follows. If you manage to pass it, the music will be out of sync.
 * In the final mini UFO part, it is also possible to skip the gravity portal by going under it.
 * There is a 'secret path' in the mini ship section after the second wave segment. You can skip the cube portal and remain a mini ship. At 80%, you will still be a mini ship. However, when you reach the cube, you will remain mini and crash because you cannot make the first jump of that section.
 * In the first ball section at around 21%, instead of skipping the third blue jump ring and landing on the ground in antigravity, if timed right, you can still hit the third blue jump ring and continue safely.
 * Right before the first wave section when you fall on to the block you have jump off to get to the portal, there is a chance you will fall and hit the side of that block instead of landing on it. This is also very unlikely to happen.
 * If you do not jump on the last blue ring on the slow cube, you will fall to the ceiling out of the world and crash as there is no sawblade.
 * At the start of the first mini ship sequence, when you are about to go through the first obstacle of two invisible spikes in an invisible block wall, you might notice the smaller one on the bottom is pushed a little to the right.
 * Also, if you crash before the first secret coin, you can see that the diagonal block on the roof does not actually connect with the horizontal block.
 * Before the first wave section, you can see that the top of the wave section is unfilled.
 * You can skip the purple jump ring in the cube section after the mini UFO if you jump the gravity jump pad late.
 * At 1 %, if you jump early, the gravity will change later because of a bug, and when you crash, you will see a missing part of a platform.
 * In the second ball segment, you can skip the second block by just sliding on the next. (You are supposed to tap and switch to normal gravity, but you can just slide, still in anti-gravity, and continue as normal.)