This site used to live at zmzm.pw, but I've since lost that domain :(
“玄之又玄 众妙之门” concludes the first chapter of the Dao De Jing. In pinyin, “xuán zhī yòu xuán, zhòng miào zhī mén” — hence ZMZM.
An ambiguity- and pun-preserving translation of the Dao De Jing’s first chapter:
道可道 非常道
名可名 非常名
The coursed course is no constant course,
The named name is no constant name.
無名天地之始
有名萬物之母
Without names the origin of heaven and earth,
With names the matrix of ten thousand things.
故常無欲 以觀其妙
常有欲 以觀其徼
Thus constantly without desiring to regard the marvelous,
Constantly with desiring to regard the mundane.
此兩者同出而異名 同謂之玄
These two, samely-arising but differently named,
Samely called dark.
玄之又玄 衆妙之門
Dark and dark again,
Door to many marvels.