Mapping the Veins of the Earth

 Navigation in the Veins of the Earth is a difficult proposition even in the best case. As Skerples puts it in his excellent Veinscrawl (link contains spoilers) supplement:

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?

This is a challenge in real life as well. Caves intrinsically resist mapping, and even with modern technology and perfect understanding of a cave (often very challenging to acquire in the first place), there is the difficulty of presentation. For mapping buildings and surface locations, top down maps or isometric maps are common because they do a decent job at matching up to how your brain wants to model that space. Applying the same logic to cave maps... 

Good luck. Even with a sophisticated approach like this that tries to depict the cave in a geometrically accurate, to-scale fashion, projecting it onto a 2D image conceals a ton of detail and nuance about the relationship of these spaces. Really you need a 3D model, because caves are volumes, voids in stone.

But medieval peasants do not have access to 3D modeling software! Perhaps one of your mage PCs has a Speak with Air spell, or the ability to perfectly understand the geometry of the space (which you can simulate by them not getting lost and being free with info), but what about the maps the players make? How does one party in a West Marches game record their findings and map a cave system for the next party? Patrick Stuart faced this same challenge when creating Veins of the Earth. From the section on generating diagrams of cave systems:

The methods chosen to do this are deliberately abstract. They are NOT LIKE NORMAL TOP-DOWN MAPS. They are not related to the surface world or to the depiction of things on the surface world. They show RELATIVE distances and the relation of things to each other underground. They are more like the map of the London Underground, or a circuit wiring diagram, than a normal map. I can’t draw, so making a system that I can’t use would be dumb. They are meant to be made fast during play. Other systems produce ‘flat’ maps, like dungeon maps which favour movement in a single plane. I wanted verticality and interlocking three-dimensional complexes.

He goes on to detail a "good enough for TTRPG purposes" system of notation which breaks systems into caves and passages, and then essentially arranges them as a pointcrawl. Caves are depicted as diamonds with feet and a hat, so that lines representing passages can connect to 6 different points and thereby show if a passage leads off from one of the four cardinal directions, the "floor", or the "ceiling". This is still imagining caves as rooms, which really, really does not do them justice as the above image alone is probably enough to suggest, but I think this notation is really cool and it is the sort of analogy to the familiar the PCs will probably be making instinctively.

Inside each diamond you draw a symbol to show generally how large the cave is. Patrick suggests a series of symbols (see the chart below) but mentions that these can really be anything if the group finds a different system convenient.

Lastly, the passages are drawn with different lines to indicate the dominant nature of the passage as relevant for human traversal. Is this a passage I can walk/climb through, or do I have to crawl? Is it full of water? It's probably full of water. He suggests four categories of passages:

• Walk/Shaft (solid line): a passage you can walk upright or crouching in, generally anything > 4ft wide/tall. If this is a vertical passage, then you can climb or abseil it without your pack rubbing against the opposite wall.
• Crawl/Chimney (dotted line): a passage that requires getting down on all fours, or if vertical, is tight enough that you can press your back to one wall and feet to the other side. Brutal to fight in here, and corpses likely obstruct passage.
• Squeeze (zigzag line): a passage < 2ft wide/tall, which requires slithering on your belly and inching your way forward. If it inclines down, both headfirst and feetfirst are unattractive choices for their own reasons. The passage is too tight to pull a pack past you, and a dead body will obstruct the passage and possibly be impossible to get past without pushing it all the way to the end or pulling it back to the start.
• Special (looping line): a route that requires traversing a chasm, swimming underwater, clambering through a breakdown pile, or some other special kind of movement. You can mark the passage with a letter or word indicating the special type (S = sump, T = traverse, B = breakdown, etc.). Probably (and this is becoming a theme here) also awful to fight in.

So yeah, passages are the scary parts of cave systems. Passages are where ambushes are the most brutal, and if the enemy somehow manages to coordinate an attack from both sides of a passage... you are probably screwed. Passages can be marked with a number to indicate how long they are, and the examples in the book assume the number means how many 10 minute long "dungeon turns" they will take to traverse on average, which is helpful for mapping to the cadence of encounter checks many OSR systems use.

Anyway, the above notation creates maps that look like this, which are actually pretty quick to make and fairly readable once you're familiar with the notation:

I see a gargantuan cave towards the bottom left with a walk coming off the south wall, and a breakdown pile in the floor which eventually leads to a house sized cave with some difficult squeezes coming off it. Seems like a decent spot for a secret base (depending on what the cave is actually like inside)! The passage in the south wall can be the "main gate", with the breakdown pile essentially as an escape route to the defensible "inner keep".

Well, I hope that this has been intriguing, and that you will join me in daily prayers for the publication of Veins of the Earth Remastered. Iä! Iä! False Machine fthagn!

To the players in my current campaign, consider using this mapping system! As we spend more time exploring certain hexes in detail, I will generate cave systems so we can do some "zoomed in", dungeon-scale play in those hexes if it becomes relevant (such as in the case of a siege or larger clash with a faction).