The machine graphics for LavaBlock.

This mod adds nothing to the game. It declares no prototypes and runs no
Lua — Factorio loads it with a checksum of 0. Install LavaBlock, which
requires this and will pull it in for you.

Why it is a separate mod

The pictures were 74 MB of a 76 MB mod. That meant a release which
renumbered one recipe, rebalanced a machine or fixed a line of English cost
every player the whole set again. Split out, a LavaBlock update is about a
megabyte and a half; the machines are downloaded once and stay put.

The two are versioned together, so keep both up to date.