Here’s a full “2025-style” guide to the Underground in Pokémon Diamond / Pokémon Pearl — or rather how the Underground works nowadays in the modern remake versions Pokémon Brilliant Diamond & Pokémon Shining Pearl — often referred to collectively as the “Grand Underground.” I’ve combined core mechanics, tips, and strategies to help you get the most out of it.
✅ What is the Grand Underground & how to access it
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The Grand Underground is a subterranean network beneath Sinnoh that you can access after getting the Explorer Kit from the “Underground Man” in Eterna City.
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You can enter from many surface locations. Where you use the Explorer Kit determines which part of the Underground you’ll appear in — but whenever you leave and re-enter, you return to your original surface exit point.
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The Underground is optional, but very useful: you can dig for rare items, find fossils, catch Pokémon in “hideaways,” build a Secret Base, and more.
🕳️ What you can do underground — Core Activities
Dig for treasures
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As you roam tunnels, small sparkling marks will appear on the map (and on walls). Press R (or the indicated button) to reveal diggable walls.
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Upon interacting, a prompt will show how many items are hidden (often 1–4). Then you enter a mini-game where you use either a Sledgehammer (Mallet) or a Pickaxe (hammer) to break the wall.
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The Sledgehammer clears more area but reduces wall stability faster.
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The Pickaxe is more precise and safer for delicate digs.
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You collect any treasures fully uncovered — but once the crack at the top reaches the left (i.e. wall collapses), dig ends.
What you can dig up:
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Spheres (currency), shards, Plates (special held-item plates), Evolution stones, held items, gems, rare items.
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Fossils: In the original DS-era Underground (Diamond/Pearl) you could find Skull Fossil (Diamond) or Shield Fossil (Pearl).
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In the remake Grand Underground (BDSP), after getting the National Dex, you can also dig up other fossils (Helix, Dome, Old Amber, Root, Claw), which you can revive into Pokémon at the Mining Museum in Oreburgh City.
Catch wild Pokémon in Hideaways
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The Grand Underground adds “Pokémon Hideaways” — caves and caverns underground where wild Pokémon roam.
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There are many hideaway “types”/terrains (Grassland caves, Icy caves, Whiteout caves, Riverbank caves, Sandsear, Spacious caves, etc.), each with a different pool of possible Pokémon.
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Some Pokémon in Hideaways are normally unobtainable (or very rare) on the Sinnoh surface at that stage — the Hideaways offer a unique opportunity.
Build & decorate a Secret Base
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You can create a Secret Base by using a Digger Drill on a wall inside tunnels.
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You decorate it with statues obtained from “Stone Boxes” (pretty boxes, gorgeous boxes) dug from walls.
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Statues have types (Standard / Color / Legendary) and their type influences which kind of Pokémon are more likely to appear in hideaways — so you can “bias” the spawn-pool based on your Secret Base’s decorations.
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You can also move your Secret Base: by acquiring another drill (from sphere-traders) and re-placing your base elsewhere — while keeping all decorations.
Trade with Underground merchants
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Scattered throughout the Underground are NPC “Hikers” / Sphere traders (represented as colored dots on the mini-map: red for selling for spheres, blue for traps, yellow for goods).
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You can trade the spheres and other treasures you dig up for useful items (e.g. Balls — Quick Balls, Dusk Balls, Dive Balls; drills; other helpful stuff) from these merchants.
Use the “Diglett / Dugtrio sparkle” bonus
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Occasionally, as you move through tunnels (especially at intersections / corners), you might encounter a wild Diglett or Dugtrio, who will run away but leave a pink sparkle behind.
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Collecting these sparkles fills a gauge (out of 40). Once full, you trigger a temporary “boost” — for a short time, your odds of uncovering rare treasures (like Gorgeous Stone Boxes with Shiny-statues) improve significantly.
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This bonus lasts only a few minutes, so it’s best to dig fast while the buff is active.
⚠️ Tips & Best Practices — How to get the most out of the Underground
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Start with Sledgehammer, then use Pickaxe: Because the wall stability decreases faster with the big hammer, it’s often safer to use the big hammer to “clear rough area” and once you detect where treasures might be, switch to Pickaxe for precision. Watch for metal hits: When your tool strikes a metal layer (or “bedrock”), wall stability drops quickly. Avoid hitting those unless necessary.
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Dig often — but don’t forget to restock: After digging and collecting spheres/shards/treasures, visit hikers to trade them, get new drills/balls/items — that way you stay stocked for further underground runs.
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Use the Diglett bonus effectively: After filling up the sparkle gauge, plan a short “digging-session sprint” — hit every sparkle-shown wall quickly to maximize your chance at rare boxes/statues.
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Decorate wisely: Tailor your Secret Base statues to the type of Pokémon you want to encounter (e.g. Ice, Water, etc.). Statues influence spawn rates — so this is a strategic lever.
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Explore many hideaway types: Because different caves have different Pokémon pools, visiting multiple terrain-types increases your diversity and chances of rare or desired Pokémon.
🎯 Why use the Underground — What makes it worth it
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Access to rare treasures and items (Fossils, Plates, Evolution Stones, held-items) that are difficult to get otherwise.
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Ability to catch Pokémon not normally available at your current storyline progress, including potential for rare / stronger Pokémon earlier.
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Build & customize a Secret Base — many players enjoy collecting statues, customizing, and even using the base for multiplayer visits.
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For collectors: full access to many Pokémon and fossils; and for competitive or late-game play: opportunity to snag rare held items / plates / evolutionary stones early.
📝 Quick FAQ — Common Questions & Answers
| Q | A |
|---|---|
| When can I start digging? | After getting the Explorer Kit from the Underground Man in Eterna City. |
| Do I need to do anything special to dig? | Walk around tunnels, look for sparkle spots on mini-map or on walls, press R to reveal diggable walls, then choose to dig. |
| Can I get fossils? | Yes — fossils (Skull/Shield originally; more fossil types after National Dex) can be dug and then revived in Oreburgh. |
| What’s the benefit of Secret Bases? | Decoration + statues can influence which type of wild Pokémon spawn. Also fun for personalization / trading. |
| Does Underground reset or refresh? | Hidden item spots reset whenever you leave and re-enter the Underground or exit a Hideaway, so you can dig repeatedly. |