This is an excerpt from Kaikhosru Sorabji’s Opus Clavicembalisticum of 1930. The snaky line running through it is a slur (!) encompassing the whole complex passage.

Indiana University information scientist Donald Byrd observes, “It has a total of 10(!) inflection points; it spans three systems, repeatedly crosses three staves (this is also the most staves within a system for any slur I know of), and goes slightly backwards — i.e., from right to left — several times.”

More at Byrd’s Gallery of Interesting Music Notation.