The 5.7 update introduced the new lava feature. Low areas on a world start forming lava oceans at a temperature of around 1,500,000mK. The hotter the world, the more the lava level rises. Above 2,000,000mK the whole world is lava.
Cooling the world gets rid of the lava, but the world's landscape will be forever altered. The areas that used to be lava will be a grey color (this does not always seem to happen), and their elevation levels will also have changed.
You can find lava worlds in the random planet generator, or you can heat up a regular world to create lava.
City Melted[edit | edit source]
[City] has been submerged beneath lava on the melting surface of [World]! [number] people were killed, and the city has been completely destroyed.