Lithodermis

    The color in the player's skin slowly grows darker in patches until he/she looks a light
    blue-grey. He/She claps, producing a rocky sort of thud.
    [ You've developed the 'lithodermis' mutation! ]

Lithodermis is the upgrade to High Density. Your skin becomes more dense and stony, helping you resist broken limbs, knockouts, beatings, falling, and brain damage. However your added weight impairs climbing and swimming, and lets you carry less weight in gear.

Location

In Adamant Canyon. From Luskentyre Plateau, go down and travel southerly within the depths of the canyon until you come upon the mouth of a cave. To be more specific, go down into the depths of the canyon and south, then southwest and south, until you can go down to the breach. From here, the map cuts out, but you want to go southwest then south for a while (there's a small river), and after crossing a rope bridge going south a bit more then east a tiny bit and you'll be there. The only real foes are rock leeches and the occasional ghost. It's a good idea to set ropes down on every climb, since with Lithodermis you'll likely be overweight and/or too low in climb to get back. Alternatively, you could just run through naked and grab the mutation then suicide by drowning or such.

Effects

This mutation grants:

    -3 to Climb
    -3 to Swim
    +70 generic broken limb resistance
    +60 knockout resistance
    +50 brain damage resistance
    +1-2 beat resistance
    +2-2 bullet resistance
    +25kg to player's weight (12.5kg to carried weight)

Advantages

This is a decent choice for characters with low Endurance, who'd otherwise easily suffer knockouts. The added beat and bullet soaks make for OK defense against many mobs, and the knockout and broken limb resistance is good to have, especially for PvP. Headbutt is also less of a problem (no matter which end of it you're on). Plus resistance to brain damage is good for those with no friends to perform surgery on them.
Another cool advantage is that fall damage is reduced noticably, so your climb handicap will rarely get you killed when you fall. Keep in mind that you can still land on your head, or your armor might take damage.

Other unlisted bonuses are a bonus to damage when using your bare fists as well as the inability to be tattooed by a tattoo gun! You can still get that tramp stamp you've always wanted by using an engraving tool in place of a tattoo gun. This can be seen as an advantage since engraving tools are extremely cheap compared to a tattoo gun, but this generally isn't a factor in taking this mut.

Disadvantages

Knockout resistance aside, the overall bonuses are nothing mindblowing.

The -3 to Swim and Climb are annoying but no big deal, as those skill raws are very cheap and plenty of people go for Salamander. Also, you can't get Flight if you have Lithodermis.
Litho adds 25kg to your weight (annoying in planes sometimes), which also makes you able to carry 12.5kg less (it's halved since it's "worn" weight). This extra weight is not shown in the inventory screen, which takes a little to get used to.
If you're a brawny character, it's hardly much of a problem, but others may want to pick up mule, as the 30% bonus to carry almost perfectly matches the 28% or so that Lithodermis can add. That also makes it nigh impossible to take stress from Mule by not carrying enough, given your passive weight.

Appearance

    His/Her skin has a bluish grey hue.