The new area is a huge cavern packed with lapis, iron, gold and diamonds, and every one of those has its own mob and its own spawner. They climb the whole thing. By the end they have more resources than they'll ever need and none of the things they came for.
Keeping the ghasts
First, the arrows from the weird skeleton in the last episode: cursed arrows with Slowness I, Weakness I and Mining Fatigue I. They decide to keep that guy and maybe build a grinder around him later.
Behind some glass there's something green. "Sure enough. That's a lot of glass. There we go, emeralds." Nobody trusts it. "This is a trap for real." Past it are lots of gravel, lots of lapis, a creeper spawner under half slabs, and a lapis-headed mob that sends one of them running.
There's also a ghast spawner in a pit, and they argue about keeping it. "Having regen potions all the time would be cool." If they block it in, the ghasts can only spawn down there. They do, and it stays.
Out of torches
"I'm out of torches. I cannot believe I'm out of torches. I had stacks and stacks." Further up are iron blocks, iron golems and skeletons. One sword breaks, and a gold sword turns out to be "not very powerful." Then a pack of ghasts. One of them takes a beating, boots and helmet half broken. "What are you up to?" "Getting blown up." They're glad the bow has Punch on it. The ghast spawner and an iron-head spawner both go down.
Someone also jumped into a lava pit by accident, then landed on magma blocks getting out. "It was very not good."
Four spawners and a treasure chest
One of the big boxes holds four spawners. Between the mobs and the loot they come away with an iron pick with Unbreaking III and Looting III, a book that may be Bane of Arthropods, TNT, and a lot of gold. A treasure chest adds a special golden apple, a Mending bow, Sweeping Edge III, and an iron helmet. "This is so cool!" Gold picks can't mine the gold here, which gets a "That's lame."
The quad spawner
Then a quad spawner, and a creeper nobody saw coming. "BasketMC was blown up by Creeper." The other one had just put water down to push mobs into a pit, and hopes it didn't wash the gear away.
Basket respawns at the start of the lava area and has to run all the way back. Getting the gear out involves a run for wood to make chests, a creeper, and an Enderman that one of them shakes off by jumping into water. A zombie is now wearing Basket's shield and boots. Scoti_Garbidis makes the advancement Cover Me With Diamonds along the way. The gear ends up in two chests, a little mixed up, and gets sorted out. "I just died, but I haven't died in a while."
All the way up
Back together, they clear the quad spawner and the iron spawner, then cross a bridge with Endermen and blazes on the pillars above. A gold mob hits hard, and a diamond one drops diamonds, which a gold pickaxe can't mine. "Classic Scoti." A special box next to where they think the wool is holds gold, iron and coal. "We have an infinite supply of diamonds now."
They reach the top without the wool or the emerald. The best guess is a structure with iron bars and blaze spawners, which means fire resistance first. They call it there.