Marvel: Superhumanly Dense Tissue

Cain Marko (Juggernaut)
Juggernaut's skin, muscle, and bone, are exactly four times denser than the same tissue in the body of a human being, greatly contributing to his superhuman weight.

John Proudstar (Thunderbird)
Thunderbird's muscle tissue was three times as dense as that of a normal human being's and was distributed in such a way as to give Thunderbird massive shoulders, arms, and thighs. His skin was several times as dense as that of a normal human being's and included an additional layer of leathery epidermis.