Common Behaviors Edit
All gaseous fluids added by Harder Underground will attempt to balance their flows (unlike standard fluids, which have a constant and even downward slope), which can allow a large volume of gas to spread very far indeed. Gasses will also attempt to flow past standard non-solid blocks that block water, such as ladders, signs, and open doors and trapdoors. Closed doors will block the flow, predictably.
Breathing gasses (being inside their volume) will slowly deplete entity air meters and eventually cause nausea, non-potion blindness, and suffocation. Due to the way vanilla does drowning, air bubbles on the status hud will not be displayed. This makes hard-to-see gasses particularly dangerous.
Every entity has a "lung volume" calculated from their volume, so chickens will suffocate sooner than players; take a mining canary with you! Undead monsters, Iron Golems, and snowmen do not need to breathe and will be unaffected.
Volatile Gas Edit
Volatile gas (or explosive gas) comes in two varieties: Thick and Thin
Thin volatile gas is lighter than air, floats, and is almost invisible. It's also incredibly flammable and prone to exploding. This gas is the primary cause of any explosions audible shortly after worldgen.
Thick volatile gas is heavier than air, sinks, and is not terribly flammable. It will flow into fires and put them out, although fires along its top surface tend to stick around for a very long time. While it does not set on fire very easily, it may explode if an explosion happens nearby, just like one exploding TNT can set off others.
When Thick gas has empty air above it, it will turn one unit of itself into 8 units of Thin gas in that (previously) empty space. This conversion can also happen if the space above is a single unit of Thin gas (in which case only 7 new units will be generated, and the 8th lost).
Both types are replaced (displaced) by water flowing into the area. It is possible to clear much of a gas vein by repeatedly placing and picking up a bucket of water.
Lava and fires will spawn smoke any time a player is in vicinity (32 blocks horizontal and 16 vertical). Smoke, which is lighter than air, doesn't "live" very long, so it doesn't flow very far (the limited radius around the player prevents smoke from causing unnecessary block updates). But it will collect in confined spaces and make navigation in any area with lava difficult.
In addition to the standard effects from breathing any gas, smoke inflicts the Blindness potion effect that limits the player's viewing distance (at about double the distance as being hit with a long-duration blindness, possible due to inflicting only 19 ticks worth of duration continuously).
Spawns in the Nether, and aside from the block updates it causes, is not as big a deal as it would seem (due to the limited lifespan, short spawning radius, and high cavern ceilings).
Rock Dust Edit
Rock dust spawns whenever unstable stone collapses. Aside from not being a fluid, instead moving about semi-randomly, it acts like any other semi-breathable gas. It will rapidly deplete a player's lung capacity and cause damage, but will typically dissipate before causing enough damage to kill a healthy player, even if there is no escape.