This drawing shows a tunnel running underneath the Playboy Mansion to the "residence of Mr J. Nicholson." It's one of a couple blueprints found in the unfinished basement of the Mansion in Holmby Hills that also shows underground tunnels to the homes of "Mr. W. Beatty," "Mr. J. Caan," and "Mr. K. Douglas." Warren Beatty, James Caan, Kirk Douglas, and Jack Nicholson were all in the neighborhood in the late 1970s and early 1980s, which is the approximate time that these passageways are said to have existed—if they existed at all, says Playboy (via BLDG Blog).
The proposed network was discovered when an editor at Playboy.com uncovered Polaroids, dated 1977, showing some serious excavations at the Playboy Mansion. When asked about the dig photos, the Mansion's general manager casually responded, "that's probably when they built the tunnels in the 70s," like it was totally common knowledge that these celebs, at the height of their stardom, had private paths onto Mansion grounds.
The search for truth carried on in the unfinished basement of the Playboy Mansion, where a few undated blueprints were found showing plans for the underground walkways. The drawings do say "conceptual" on them, and seem to have been made fairly early in the process. But they also show that the idea for these subterranean routes got far along enough for someone to think seriously about how to connect Warren Beatty's house with the Mansion. Beatty's house, according to the plans, was on the opposite site of the Los Angeles Country Club, on Sunset Boulevard—the tunnel would have had to go under the golf course and part of Sunset.
In the end, no one—not even the initially helpful general manager—will say whether the secret underground network was ever built, though an anonymous staffer did say that they'd "heard [the tunnels] were closed up sometime in 1989." There are more blueprint and excavation photos over at Playboy.com.