Sonic Mania/Oil Ocean

Oil Ocean is the seventh zone in Sonic 2 and as indicated by the name, is an ocean that has had oil dumped into it by Dr. Eggman. Though sinking into the oil isn't generally recommended, if you sink into it in the right area, you'll be able to find a hidden area that unlocks an achievement/trophy, as is explained by the walkthrough for the second act.

Act 1
The first act should be straightforward and fairly quick. Steam vents that burst can lift you up into the air, balloons can shoot you around the map, and oily slides can send you down into new areas. To the right of the second steam vent is a Fire Shield, which will set all surrounding oil on fire (at no cost to your health) and deflect projectiles from enemies. A fan will attempt to stop you from boarding a lift; to get past it, a running jump should suffice. After transferring lifts for a while, you should come across some slides and, across from the first slide, the first Star Post.

Eventually, you should come across some spiked rings that slide across the ground. Avoid making contact with them, as you won't be able to do any damage to them. Past this, jump to the right from the crumbling platform to reach an Extra Life. Drop down to find the second Star Post. The act's boss is waiting a short distance from here. Start by jumping at him from an angle (if you attack from under, you can get caught by his propeller). If you aren't able to get six hits in quickly enough, he'll turn a valve that triggers the steam vents; stay toward the center to avoid being shot upward into some spikes.

After defeating the boss, the area will light up in flames, setting up the environment for the next act.

Act 2
The area can now become toxic enough to the point that it becomes a hazard, with visibility becoming limited and your ring count steadily decreasing. To temporarily clear the air, you must find hanging switches scattered throughout the stage. If you're in search of the secret submarine for the achievement/trophy, start by crossing the crumbling bridge and climbing the oil waterfall (by jumping repeatedly) to reach a lift to the right. Run up the ramp (you can push into the spring on the left for a boost), then hop into the first tube to be shot into some balloons. This path should lead you right to the second switch. Continue to the left, springing up into another oil waterfall. From here, jump into the next waterfall, then on ground to the right. Jump up one more oil waterfall to continue further right, then spring into a slide. The slide should lead you to the third switch. Continue right, sliding down into the fourth switch. Hop off to a platform with a Fire Shield under it. Drop under the platform and sink into the oil directly beneath the Fire Shield. Proceed down the oil waterfall and sink even further down into the next sea of oil. This should launch you into a balloon; continue on the path it sets for you until you come to a stop. Drop down a crumbling bridge to the right to find the fifth switch. Continue left up a lift, then right across a gap and down a slide. Upon reaching the next crumbling platform, drop down it again and proceed to the left, down a ramp. Ignore the spring and continue heading right. Drop down onto a lift, but walk off of it to the right before it can send you upward. Here, you should find the sixth switch. Now return to the left and drop down the gap previously filled by the lift. You should now yet again find yourself in a sea of oil. Proceed to the right (you need to let yourself sink a bit to get under the support structure) until you see a submarine. Hop on top of it, enter the hatch, and feel free to reap the rewards inside. Upon reaching the boss, Dr. Eggman's Mega Octus (which shares some attacks with the boss of Sonic 2's Oil Ocean), hop into the oil and hit it with as many Spin Jumps as you can. To defeat the boss entirely, you must land eight hits on the main body. This becomes difficult to do when the main body sinks into the ocean, sending some of its tentacles after you in its place. Once it descends out of view, hop onto a platform and await the initial tentacle attacks, which may consist of a spiked tentacle launching itself in your direction (which you must jump over) or a tentacle slowly rising out of the ocean to shoot you with laser attacks (either shooting projectiles at you directly or shooting the platform, sending a shockwave across it that you must jump over). After dodging these attacks, you'll be able to attack the two primary tentacles (which each take four hits to dismantle), and shortly afterward, the main body. While attacking the tentacles isn't necessary, the projectiles that they shoot can become a nuisance, so consider getting a couple of hits in on one of them before the main body rises up. After a couple of rounds of this, you should be able to finish the Mega Octus off, allowing you to free this zone's animals and move onto the next zone.