Meltdown! is an achievement in Plants vs. Zombies 2. It is achieved by completing Frostbite Caves - Day 30. It is worth 20 points on Game Center for iOS devices and 2,000 XP on Google Play Games for Android devices.
Its name is possibly a reference to the 2006 movie Ice Age: The Meltdown made by 20th Century Fox. However, the real Ice Age ended with a meltdown, so it could also be a reference to the latter.