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The Underground is a separate map in each campaign of Age Of Wonders 4. It can be difficult to navigate because of the rough terrain, but taking the time to explore the caves can earn you extra treasure, troop experience, and even some great city locations!
If you've been avoiding the places below, never fear - this guide will teach you everything about searching the Underground. You never know what you may find until you look, after all. Read on and you'll be ready to become the next Ruler of the Underground.
How To Reach The Underground
Scattered across the map are Underground Passages, caves that connect the surface world with the Underground. All Underground Passages are in connected pairs, relative to one another's position on their respective maps. Even if the Underground is larger or smaller than the surface, a Passage in the northwest corner of the upper map will put you in the northwest corner of the lower map.
To pass through an Underground Passage, a stack must have enough movement points to travel as though the two Passages were adjacent. You'll see a popup when a stack enters a Passage, letting you confirm that you want the troops to go through. If they don't have enough movement, the button will be grayed out.
You can also click on the tile that the Passage is on while a stack is present to make the popup appear.
If a stack attempts to pass through an Underground Passage while an enemy group is on the other end, a battle will trigger. The units waiting on the other side will be considered the defenders, and will be able to call reinforcements from nearby stacks. The attacking stack can only call reinforcements from friendly units that are already on the other side, so entering a defended Underground Passage is dangerous!
How To Excavate Underground Regions
Early in the General Affinity tree, you can unlock the ability to excavate the Underground. The icon for the skill is a shovel, and once unlocked your units will get a similar icon on the right side of their portraits on the world map; it's next to the button used to toggle road building on and off.
Clicking the Excavate button will show which Underground walls nearby can be dug up; they'll have markers on them similar to those used to denote Province Improvements. If a stack is adjacent to such a wall, you can click on the shovel icon on the marker to start excavating. This uses up the rest of the stack's movement points for the turn.
At the start of the next turn, as long as the excavating stack is still there (that is, they haven't been destroyed or disbanded), the wall will disappear, revealing what was beneath it. Sometimes there will be nothing but more open space, which can be used for traversal or building. Other times, you'll find treasure, hidden tunnels, or hostile Marauders.
The Best Way To Navigate The Underground
Traveling through the Underground can help you get troops from one place to another without having to cross oceans or pass through hostile territory. There aren't always straightforward routes, so the best way to travel is to use the many rivers in the Underground.
Once you've planned out a route, send a small army to build roads ahead of your main force. Underground terrain is rocky, so it can slow most units down considerably. Having a Hero along to build Outposts, which can then attach Harbors with a Work Camp, can ease embarking and disembarking on the rivers.
Underground Cities
The Underground is, unsurprisingly, great for cities with lots of Quarries. This lets you construct lots of buildings and set yourself up with an out-of-the-way spot for victory provinces. Other resources, notably Food, are less plentiful in the Underground, so it's best to use the extra construction from your Quarries to build structures that can make up the difference.
If a city's species has the Underground Adaptation ability (Molekin have it by default) then you can build Farms on Underground terrain, speeding the growth of subterranean cities.
Underground cities tend to have fewer provinces than ones build on the surface, so placement is important. The Underground usually has several Silver-tier Ancient Wonders, so settling near one of those is usually the best play. Also be sure to look for fungal forests or gold and iron deposits to enhance your new town!
Of course, there are dangers below as well. Infestations aren't necessarily more common or higher-tier in the Underground, but the tight quarters can make them more of a problem. You also want to be sure that your new city isn't too far away to defend, so it's best to control both sides of the nearest Underground Passage, ideally from within your domain.