OSR: Navigating Underground

The worst case scenario for a surface hexcrawl - fog at night, no map, unfamiliar and dangerous terrain - sounds like the best case scenario for an underground cave-based hexcrawl. You can't see terrain in the distance. You probably don't know north from south. You can barely tell where you've been. Hostile forces surround you. How can you possibly navigate in this mess?

 ExitStageLeft

Location-Based Goals

The PCs find a clue. "The Pyre Monks know the secret," they say. "We need to find their monastery." How?

1. Directions

On the surface, directions are given with three components: direction, distance/time, and landmarks.

"It's half a day's ride to the north along the old mining road." 

"Turn south after the hanging tree and travel for another six miles until you reach the river."

Underground, directions are given with three components: rocks, depth, and landmarks.

"Pass the blue-grey marble caves, then travel down to a boulder field. When you hear the hiss of flame, you are close." 

"Climb to reach the Ghoul Baron, but pass beneath his fortress quickly. Follow quartz, then basalt. Descend again and search for the smell of smoke."

All hex descriptions list a rock type and a depth (littoral, profundal, and abyssal). Time is meaningless. A journey might take one group six hours. Another group, following a slightly different path, could take six days. Travelers, wanderers, elders, and experts might be able to give PCs directions, or direct them to someone who can. Trading gold for directions is important.

2. Water

If you are lucky, the thing you are looking for is near a major river or lake. Follow the water.

3. Paths

There are known paths in the Veins. Follow Olm tactile-marks to find an Olm camp, a Dvergr mine to reach a Dvergr operation. Volume-Folk pitons and rope. Illithid tunnels. Drow causeways.

They aren't consistent. Most aren't very long, but all hexes owned by a faction have fragments of roads connecting them. If you see a cave without one, you know you are probably heading away from that faction's territory. If you take your time, you can use this to navigate through or away from a faction.

The PCs are somewhere in the hex. Roll 1d6 to see where they go.

Wandering (Blind) is for blind panic, blind drunk, or blind, hungry, and lost. Purely random. Exit the hex and end up somewhere else.

Wandering (Directed) is for PCs with a plan but no map or consistent directions. The "intended direction" is where they want to go. Extra time, people familiar with cave exploration, and a sensible plan should also decrease the chance of random error. If your system has a Navigate or a Wilderness skill, it might be useful here.

Wandering (to Faction A) is for people who want to stay within a faction's territory.

Wandering (away from Faction A) is for people who want to leave a faction's territory.

You can't wander away from a faction if you're in a neutral hex. You don't know what you're going to find until you find it.

Fleeing PCs can't plan. If the PCs want to get away from something rather than go somewhere, they are Wandering.

PCs can't cross solid black lines without a plan. If they are Wandering, adjust probabilities so they can only reach unblocked hexes. Similarly, on the edge of the map, adjust probabilities so they don't wander off the edge.

Other Methods of Navigation

Magic
Some spells point in a fixed direction. Some show a path. Entire careers built around a "Locate Dolphin" spell - underground, a fixed coordinate is a powerful tool. If the PCs can consistently (at least once per hour) find one of the cardinal directions, they can navigate using "north, south, east, and west", just like a standard hexcrawl. Even if they can only find north once per day, they can still tell if they are heading in approximately the right direction.

Entrails
Magic so old it isn't really magic anymore. Find a creature, spill its guts, look for patterns, hints, signs. Might be a psychosomatic effect. Might actually work.

Guides
Expensive. At least 10gp a day, with half paid up front and kept somewhere secure. There's no point in being rich if your treasure vanishes with you. Guides can still get lost. They have a 4-in-6 chance of taking you in the direction you want to go. On a 1 or a 6, they take you 1 hex to the left or right of your target.

Veins Maps
Dvergr three-dimensional carvings. Illithid memory-injections. Drow silk, carefully dyed. If you have a Veins map, you travel at 1/2 speed (so 12 hrs to cross a hex), but you will always head towards your destination.

PC Maps

PC-drawn maps should look very strange, more like a set of encounters or linked concepts than a true geographic map. They will barely resemble the hexcrawl map. One player should be in charge of mapping. Their artistic results will... vary.

As long as PCs can describe their path, and they aren't rushed or pursued, they can always retrace their steps. In the example above, players visiting the Olm could retrace their steps to the Waterfall or the River or the Bottle Cave (whatever those hexes might be).

If a location or encounter wasn't included on the PC's map, they cannot deliberately find it again. If they missed recording a vital landmark or location, see the Wandering rules above.

PCs need something to write on (parchment, spellbooks, their own skin) and light to read the map. If they lose the map, they can try to redraw it from memory.

Hex Scale vs. Encounter Scale

Hex travel is highly abstracted. PCs are assumed to be climbing, walking, crawling, and navigating, but the exact details of every step or every 10' segment aren't covered.

Hex-Scale Rules

Hexes normally take 6 hours to cross.
The PCs will encounter the Obvious Feature of the hex.
If they enter cautiously, take unexpected routes, explore, or delay, they may encounter the Hidden Feature instead. Alternatively, GMs can use the Hidden Feature instead of the Obvious Feature if they don't like the Obvious Feature. 

Roll on the Random Encounter Table for the hex type at least once per hex or at least once every 24 hours. 

The PCs get the Omen before the encounter unless:
-they are making a lot of noise
-they are Starving
-they are traveling quickly

The group can always choose The Rapture (VotE pg. 107) instead of an encounter before the Omen is revealed. The Rapture only targets one PC; a random encounter could claim them all.

Hiding from encounters is completely viable. 

Encounter-Scale Rules

Zoom in. The map dissolves in a pixel-by-pixel screenwipe, revealing location and an encounter (or the Omen of an encounter). All the usual rules for cave and dungeon navigation apply.

If you do not know where the encounter is taking place, roll on the Encounter Terrain Table (below) and the Cave Shape Table (VotE pg. 258). You can also use this method for Obvious/Hidden Features. Connect caves using the rules on VotE pg. 221, or add caves from VotE  pg. 263

Where They Find You

1d20	Type	Detail
Bad
1	Gallery	Long, flat.
2	Gallery	Long, gently sloped.
3	Snake	Curved, coiled.
4	Rockfall	Boulders, space between or over.
5	Bowl	High sides, low bottom.
6	Pipe	Angled upwards, just enough to walk or run up or down.
7	Scramble	Climb on hands and knees in places.
8	Ledge	A narrow path next to a deep drop.
9	Waterfall	Roaring water. Blocks all sound 10x1d10' around bottom.
10	Pools	Water up to the ankles. Hidden leg-breaking pits.
Worse
11	Rappel	Just about to start. 10x1d10' to bottom.
12	Rappel	Halfway down. 10x1d10' to bottom.
13	Climb	Halfway up. 10x1d10' to top, 10x1d10' to bottom.
14	Climb	Horizontal traverse. Finger and toe-holds only. 20x1d10' to safety.
15	Sump	Water up to waist.
16	Sump	Water up to neck. Need to duck underwater in some places.
17	Crawl	Low ceiling, cannot stand. Fight on hands and knees.
18	Squeeze	Impossibly tight. Movement is slow. Panic is deadly.
19	Chimney	Narrow, tall. Wedged in a vertical slice.
20	Dead End	Cornered.