Lava

Lava is a fluid block which flows slowly. It cannot be legitimately obtained in game, but can be found naturally.

Generation
Lava generates at the bottom of the world as a one block thick layer. Directly underneath this layer is Bedrock, which can be seen if this layer is cleared using gravity-affected blocks or Sponges. It can sometimes be exposed by deep caves at the bottom of the world.

Lava can also rarely generate higher up as part of a flooded cave. On worlds generated using the island type, lava caves are 25 times more common.

Behaviour
When Lava is not flowing, it uses the still block ID. In this form, the block is not ticked, and remains dormant until it receives a block update and finds an empty space to flow into. The block will then convert to the flowing block ID.

In the flowing form, Lava is ticked periodically every 8 ticks, which is half of the rate of Water. Each tick, it will attempt to flow into any empty spaces around it. If no empty spaces are found, the block will revert to the still block ID.