Sinking pool

Sinking pools are obstacles found in many levels in Genesis-era Sonic games. They are pools of quicksand, oil, or other liquids that can drown Sonic in seconds unless he leaps out continuously or reaches dry land.

Sonic the Hedgehog 2
In Oil Ocean Zone, the entire bottom of the level is flooded in thick crude oil. It is possible to skim across the entire zone by Spin Dashing across the oil, but it requires very good timing.

Sonic the Hedgehog 3
Marble Garden Zone has small tar pits that Sonic can drown in. Azure Lake Zone also has one tar pit, which the player should simply leap over in order to save time.

Sonic & Knuckles
There are quicksand pits throughout Sandopolis Zone. Most of them will drown Sonic, but some of them have secret areas underneath that can be accessed if the player sinks into them. The Act 1 boss is defeated by being lured or knocked into a quicksand pit.