A highly unstable gas found in pockets throughout the Briar Patch, which Riker collected and vented at pursuing Son'a warships to destroy them in Star Trek: Insurrection.
Metreon gas is a volatile substance that drifts in clouds through the Briar Patch, the turbulent region containing the Ba'ku planet. It ignites violently when struck by weapons fire, one of several hazards that make the region difficult to navigate and to fight in.
In Star Trek: Insurrection in 2375, Riker and La Forge take the Enterprise-E out of the Briar Patch to tell the Federation Council what the Son'a and Admiral Dougherty are doing to the Ba'ku. Two Son'a ships pursue and use a banned subspace weapon that forces the Enterprise to eject its warp core.
Outgunned, Riker has the ship's ramscoops gather metreon gas and then release it in the path of the Son'a, taking the helm himself with a manual control column. When the Son'a fire, the gas explodes, destroying one ship and crippling the other. La Forge dubs the tactic the Riker Manoeuvre.