How to Beat the Underground Final Boss

What this fight is

The underground final boss is the capstone of Builder Quest 8 in Chapter 2. After you descend through Mining Island or a linked underground mining cavern, the quest marks a sealed arena at the deepest vein. The fight mirrors the Trash Bot structure — add waves, environmental hazards, then a weak-point burst window — but adds flying Minerbots that ram vehicles and spray rock shrapnel.

Completing the fight unlocks the Ore Prospector at the Trader, awards the Chapter closed Steam achievement, and triggers epilogue dialogue with Dr. Elena Voss. You cannot skip it and still finish the Survival story in 1.0.

Recommended gear

• Spud Gun — 120+ rounds primary DPS
• Spud Shotgun — Minerbot adds at close range
• Plasma Drill — mounted on a stable vehicle platform for armored phases; see plasma tools
• 6+ Revival Burgers — Cookbot and food mandatory; hunger drain continues in the arena
• Tier 3 metal or concrete — 40 blocks for bunker walls
• Connect Tool — rewire turret bearings if Cablebots latch during adds

Leave the Baby Craftbot at the surface — the arena ceiling is too low for repair vehicles.

Arena layout before triggering

Build inside the quest arena before interacting with the central terminal:

1. Circular bunker — 6-block diameter, one doorway facing the terminal.
2. Four Spud Guns on bearings — wired to a motion sensor on the doorway.
3. Roof layer — metal blocks only; Minerbots dive from above in phase 3.
4. Bed inside — respawn point; set it before triggering.
5. Lava trench marker — the arena floor cracks in phase 4; keep clockwise movement.

This layout is identical to the Trash Bot rooftop bunker but needs a roof because Minerbots attack from the air.

Phase breakdown

Phase 1 — Cablebot floor adds

Cablebots latch onto metal floors and drain batteries. Stay on wood blocks inside the bunker; let turrets clear the floor. If latched, cut power with the Connect Tool or jump off the metal section.

Phase 2 — Flying Minerbots

Minerbots enter in pairs. They hover, spray rocks in a cone, then ram the largest metal object they see. Hide behind bunker walls; do not stand on your vehicle. Counter with Spud Shotgun when they dive — one full burst staggers the ram wind-up.

Official 1.0 patch notes describe damaged Minerbots spinning out of control when hit mid-ram — use that window for Plasma Drill burst damage.

Phase 3 — Boss surface attacks

The boss uses junk-ball rolls and side-pipe shrapnel similar to the Trash Bot. Kite clockwise; never stand in the same lane twice. Turrets handle ground adds.

Phase 4 — Lava cracks and slime

Floor vents open. Lava spreads one block per second in 1.0 tuning — jump gaps, do not chase DPS through puddles. Same slime slow as the Trash Bot fight; Revival Burgers here, not in phase 1.

Phase 5 — Core window

The boss lowers its head; the glowing core appears for five seconds. Empty the Spud Gun, then Plasma Drill or Spud Launcher. Missing the window repeats phase 3 attacks — do not waste burgers early.

After the fight

Loot includes:

• Ore Prospector schematic unlock (automatic quest completion)
• Rare components for new 1.0 parts
• One random cosmetic garment (Fan Pack items can also drop if you own the DLC)

Return to Grow Labs for epilogue dialogue. The compass unlocks repeat underground farming routes without quest markers.

Solo vs co-op

• Solo — bunker + turrets mandatory; Plasma Drill on a stationary platform behind the bunker wall.
• Co-op — one player runs Minerbot kiting, one repairs turret wiring, one holds DPS on the core window.

Quest credit is shared; achievements require the triggering player to land the final core hit on some Steam builds — let the host finish phase 5.

Common mistakes

• Triggering without a roof — Minerbots wipe open-top bunkers in seconds.
• Bringing a wide mining rig into the arena — ram attacks target the largest metal mass.
• Ignoring Cablebot latch — drained batteries disable turrets mid-fight.
• Skipping Quest 7 Plasma Drill — core phase DPS is too low with Spud Gun alone.

For quest order and NPC dialogue, read how to complete early quests and quests and NPCs.