The Ice Wind Lava Caves are lava tubes Deacon explores in What It Takes To Survive, while meeting O'Brian. You meet your first Reacher here and then fight a 100-Swarmer Horde underground.
Tunnels work in your favour against the Horde. Back into a narrow passage and use fire where they bunch up.
O'Brian explains here how Hordes hold together and why NERO cares about how fast the Freakers are changing.