Vespene gas

Vespene gas (or just "gas" or "vespene" for short) is one of the two main resources of StarCraft and StarCraft II, the other being minerals.

Overview
According to Doran Routhe, a famous United Powers League scientist, planetary systems beyond Sol could contain new resources that could benefit a resource-depleted Earth. Although the terran colonists went beyond their intended destination, Routhe's theories were proven correct, the Koprulu Sector featuring resources not found on Earth, including vespene gas.

Vespene gas is usually green in color, though the color may change through contamination; silver vespene deposits have been observed. It is highly prized by terrans due to its unusually high potential energy rating and as such, is used as a fuel in high-performance engines, energy reactors and simple vehicles in a manner similar to Earth's fossil fuels. Often found in geysers, deposits of vespene can be found on many planets along the galactic rim and in asteroid fields, a number of space platforms having been built to harvest the resource. In addition, vespene can be extracted directly from a planet's crust, albeit with more difficulty. Deposits can be found via geo-survey units, courtesy of the gas's electromagnetic returns.

Purity of the vespene is highly valued. Hydrogen sulfide is one such impurity, causing the gas mixture to become explosive. Contaminated vespene has a yellowish tinge. Liquid vespene is also usable.

Vespene gas is also used by other races. The zerg have adapted themselves to use raw vespene as a source of nourishment to drive their greatly accelerated metabolisms, raw vespene exposure resulting in enhancement of the ultralisks physical capabilities. In contrast, the protoss use vespene as a catalyst for their psi-driven machines.

Vespene formations slowly regrow over time on planets such as Antiga Prime, contributing greatly to the ability of terrans to survive in the Koprulu Sector. However, while vespene geysers effectively have an unlimited amount of gas within them, after a certain point, the geyser "collapses," only trace amounts of vespene able to be found.

Vespene gas is somewhat flammable if fire of a high enough temperature is applied and explosive with sufficent firepower.

Mining / Gathering
In order to transport and store vespene gas, it must first be converted into a liquid form. Each race has a type of structure for performing this conversion: the terran refinery, the zerg extractor, and the protoss assimilator. Each geyser has a fixed amount of gas readily available (usually 5,000), and it is collected 8 units at a time and brought to the base by a worker (terran SCV, zerg drone, or protoss probe); only one collector unit can be inside the building collecting at any one time, but multiple workers can be queued up to mine from it (usually no more than 4). Once the fixed amount of gas is expended, the source is said to be depleted after which only 2 units of gas are collected at a time.

Usage
Generally speaking, the more advanced a unit is and the more abilities it has, the more vespene gas it requires to produce. Flying units and spellcasters require vespene to create. Basic structures do not require vespene gas to construct (except the hydralisk den), while advanced structures usually do. Terran units also require vespene gas to repair units that were built with vespene.

Beginning
Because a player with no vespene gas should be able to construct a base (and players start with 50 minerals and no gas), the workers, the base structure ("town hall"), and the vespene gas converter structure never require vespene gas to construct. Each race also has a simple ground soldier that requires only minerals to produce, the terran marine, the zerg zergling, and the protoss zealot.

StarCraft II Gameplay
Most maps have two vespene geysers at each starting location and at each expansion. A standard gas value is 2500 vespene gas units.

The optimal number of workers at each geyser is three (so six for both geysers). A depleted geyser will show "Gas: 0" instead of the word "depleted". Once a geyser has been depleted, no more gas can be extracted, and the collector turns a reddish tint.

Workers can pick up 4 units of gas at a time from a geyser. The amount of gas picked up per trip was reduced because workers now have much better pathing AI (so this reduces the pick up rate to approximately that in StarCraft I).

Rich Vespene
Rich vespane, was introduced in StarCraft II. They can be gathered faster than regular vespene, and provide 6 vespene per trip.

The standard size of each rich vespane geyser is 2500 units. It is unknown if they appear in the StarCraft II Beta, or if they only show up in the StarCraft II Editor.