Mega Mack

Mega Mack, also known as megamalatricarboSonichloride, is the name given in some continuities to the pink liquid found at Chemical Plant Zone in Sonic the Hedgehog 2. According to the English instruction manual of Sonic the Hedgehog 2, Dr. Robotnik flooded the factory with a pool of the poisonous liquid to poison Sonic in just a few moments time.

Sonic the Hedgehog 2
In Sonic the Hedgehog 2, it has the properties of water (apart from its color), meaning that Sonic or Tails can drown after being submerged for extended periods of time. The Mega Mack functions like regular water so it doesn't poison Sonic in game but there are no air bubbles in the liquid so it is easier to drown in. In fact, the Japanese manual considers it water with an odd coloring.

The Chemical Plant Zone boss, Water Eggman, uses Mega Mack as part of his attack pattern. He will fill up his canister with Mega Mack and attempt to dump it on Sonic or Tails. Additionally, the rotating platforms that the battle takes place on will dump them into the Mega Mack, which will result in an instant death.

Sonic Generations
Mega Mack also appears in the Sonic Generations version of Chemical Plant. When Tails is saying that the place looks familiar, he also mentions that the "pink water" makes him nervous. This is a reference to the infamous section in Sonic the Hedgehog 2.

Sonic the Comic
In Sonic the Comic, Mega Mack is revealed to actually be a poisonous substance. It was designed by Doctor Robotnik and another scientist specifically for the purpose of killing Sonic. Sonic can not only drown in it, but prolonged exposure will actually poison him (and, indeed, he admits to feeling slightly dizzy after standing in it for a short time).

The second scientist who worked with Robotnik to invent the Mega Mack was caught in a chamber that flooded with Mega Mack, but instead of dying he was transformed into the villain Megatox, whose body is made up entirely of the substance.

Archie Comics
In Archie's Sonic the Hedgehog comics, the Chemical Plant Zone is featured in the "Sonic Genesis" arc. As in the game, when Sonic and his company arrived to the section where there are shifting blocks, Mega Mack would flood the place. But they managed to escape. However, it is referred to as Mega-muck in this story. (StH #228)

Mega-muck would later reappear while Sonic, Tails, and Mega Man went through a Special Stage half-pipe during the Sonic the Hedgehog and Mega Man comic crossover event, Worlds Collide. Surrounding the Mega Mack was a desert and several robots from the Mega Man universe. (WC #8)

Trivia

 * The water in Scrap Brain Zone Act 3 in the original Sonic the Hedgehog is colored pink, like Mega Mack. In Sonic the Hedgehog CD, the water in Palmtree Panic and Tidal Tempest's respective Bad Futures is pink as well.