Increases cluster yields from mines by 4x or more, and causes pollution. Currently, the only way to learn the skill is to have another player teach it to you (2 lessons), though teaching methods are different from other skills.
Potential teachers of enhanced mining can be classified into three tiers. The first tier is the person who initially learned the skill (from a mysterious altar). A tier 1 teacher can teach a limited number of people (unknown at the moment, but believed to be seven).
The people taught in this manner are now tier 2 teachers, and can also teach multiple people (believed to be three).
Tier 3 teachers can only teach one person, and once they teach that person, they lose the skill and become "muddled," meaning they cannot be taught the skill again.
People taught in this manner are considered tier 3 teachers (i.e. they'll become muddled if they teach someone else).
Initially, cluster yields from mining will be multiplied by a factor of 4. Note that you can still get blanks from mining (4 * 0 = 0).
As you mine, the factor appears to slowly increase. In other words, you may notice your cluster yields being increased by 5x or more. When this happens, your skill level is considered to have increased, though your skills menu will still show the skill at level 1 (this may or may not be a bug, and thus is considered a more informal classification). It is unknown if the increase happens at a fixed number of pulls, is random, or a combonation of the two (i.e. a minimum number of pulls, say about 200, plus a random number).
Currently, the highest yield is 8x, and it is unknown if it increases beyond that. If it does, then there may be no cap to how high the mining level may go.
There has been two types of pollution observed to be associated with enhanced mining: Heavy metal pollution, and purity drop.
If a mine has been extensively mined, the surrounding area may show heavy metal pollution. Pollution radius has been observed to be around 80 to 90 coordinates. So far, the only observed effect is killing flax yields, so if you plan on growing flax, it would be a good idea to do a heavy metals ecology test before growing. Heavy metal pollution seems to clear up fairly quickly.
I think it's still best to check in future with the nearby community before doing such activity. Suddenly, the flax area where I usually do flaxing where our guild flax equipment is, is suddenly still heavily polluted. - Myremi
Due to recent concerns about heavy metal pollution, I've begun doing more formal research on it. Current results are at Heavy Metal Pollution Tests. -Orashin
A more serious pollution, the purity of the mines used decrease and clusters begin turning into sand. After a certain number of pulls (the number depends on your enhanced mining level - the higher it is, the less the pulls needed), the mine logbook will note a purity of less than 100%. It reports levels as being 96%, 91%, 84%, 75%, 64%, 51%, 36%, 19%, and 0%. Note however that the actual purity may be different from what's reported.
Impure mines will turn some clusters on all rocks into sand. A rock from a 96% pure mine will have 4% of it's clusters be sand. At high purity levels, this drop in purity isn't even noticable to normal miners, and may only be spotted by enhanced miners with high enough skill level (in this example, only a rock with at least 25 clusters would have any sand clusters).
After the 96% level, the number of pulls it takes an enhanced miner to drop a mine to the next level is roughly half the number of pulls it took to drop the mine to 96%. Because the levels are spaced further apart the lower they go, this means that continued mining will drop the purity level of a mine faster and faster.
It takes 250 pulls +- 5 for normal miner to recover a mine one level. It takes 41-42 pulls for 7X yield EMiner to drop each level, also it takes same EMiner 84 pulls to drop a virgin mine to 96% (first drop). So, it takes roughly 6 times as many pulls by a normal miner to counteract EMiner with 7X yield. And the first drop takes twice as many pulls (as if there is another, invisible level between 96% and 100%).
Purity can be restored by getting normal miners to work the mine (it is currently unknown if muddled miners count as normal miners). Mine restoration seems to work similarly to purity drop - initial levels are restored fairly rapidly, but later levels take more effort.
Constant purity can be maintained by getting normal miners to work with enhanced miners. The current suggestion is to pair one normal miner to an enhanced miner for each level of enhanced mining.