Class: the Miner

A class of specialists in digging, excavation and other underground activities. Largely self-explanatory, fluff wise. This is the grubby, burly expert you bring along on your expedition when you're expecting serious subterranean work will need doing. By turns a caver, engineer, labourer and prospector.
Design wise, the class is intended to be good at dealing with the practical problems of underground travel: they can dig quickly, cope in the dark, wriggle through gaps, and so on. In terms of combat, they get to use any armour and have a good hit-dice, meaning they're at home on the front line, although not as skilful as fighter types.

Experience: The miner levels up at the same speed as a Cleric.
Hit Dice: d8
Attack chance: as a Cleric
Saves: as a Cleric
Weapons & Armour: if your system restricts weapons & armour, then the Miner can wear any armour, but may not use shields. They can use any melee weapons, but the only ranged weapons they are allowed are thrown weapons.
800xp
Spot Underground Features: Miners have a 3-in-6 chance to spot underground features, which increases to 4-in-6 at 4th level, and by 1 every 4 levels after that (at 8th, and 12th). This lets a miner spot features such as unsafe construction, traps concealed in underground structures, geological features, the culture that likely built an underground environment, hidden exits, areas that can easily be collapsed, thin easilly-broken walls, seams of metal ore or gems, and so on. This is essentially the same as a Dwarf's stone-senses and an Elf's ability to spot hidden doors combined.
Efficient Worker: A miner working underground can accomplish much more than other single characters. When tunneling, building underground, excavating and so on, the miner's efficiency is multiplied by their level plus 1. So, at level 1, they are as efficient as two normal workers combined, at level 2 they're can do as much as three workers, etc.
Low-light Vision: A miner can see further in dim light than normal. Multiply the distance they can see by their level plus 1. So, at level 1, they can see twice as far as normal, at level 2 they can see three times as far, and so on. Similarly, since they're used to working in the dark, they can always find gear stowed on their person, light a lantern, tie knots and perform similar tasks from memory, without needing to be able to see at all. Darkness has no effect on their ability to perform a task that only involves themselves and their own equipment.
Spelunking: A miner is better at moving underground than other characters. They have a chance to wriggle through any gap at least 6 inches by 6 inches without so much as a scrape. They can likewise climb and crawl over steep, slippery or unstable surfaces that other characters would be unable to tackle. Their chance to succeed here is 3-in-6, which increases to 4-in-6 at 4th level, and by 1 every 4 levels after that (at 8th, and 12th). This is essentially like a thief's ability to climb, with added bonuses to wriggling.
Pick Expertise: A miner is an expert at using their pick-axe, even when the stone they're mining fights back. When using a pick-axe to fight monsters made of stone, clay, metal, and so on, they ignore any damage resistance or reduction that monster has. Their pick-axe always does its normal damage against things made of stone, metal, etc and is never halved, reduced or ignored. (Treat a pick-axe as a two-handed axe).