Slime rain

Info
The slime rain is a rare event that causes the mass spawning of Pre Hardmode slimes, usually in a forest biome. It cannot be triggered by any item, nor by talking to an NPC. It happens completely randomly when at least one player in the world has one hundred and forty HP and eight defense, or more. The slime rain will continue indefinitely until you defeat 150 slimes, or 75 if King Slime has been defeated. When you manage to kill this amount of slimes, King Slime himself will be summoned and continue to attack the player. Summoning King Slime this way makes no difference to the fight, except from the extreme amount of slimes being spawned due to the slime rain. Once King Slime is defeated, or you are killed by him, the slime rain will cease and the Terrarian day will continue as normal.

A slime rain is a great way to harvest gel, so using a battle potion and/or a water candle is greatly advised. Though be careful for king slime, as he will spawn after the slime kill threshold. Another great way to farm gel is using the Slime Statue and wires purchased from the mechanic.