This article, Density Shifting, can be used freely without the creator's explicit permission
Density Shifting
Known Users Amazo (mimicking Martian Manhunter)
Green Beetle
Martian Manhunter
Miss Martian

Density shifting is a skill that Martians can master to "ghost" through solid objects, although it will fail if the object's density changes. It is an advanced technique, which took Miss Martian up to five years of practice.