If tombstones could sing, they would sound like ''Unearthed,'' a boxed set of primarily unreleased Johnny Cash tracks from the last decade. The analogy refers in part to the packaging: With its five discs and 104-page booklet, the box has the heft of a small headstone. But it also applies to the music. Recorded during a period of time when Cash's long-suffering body was finally beginning to fail him for good, ''Unearthed'' is the sound of a subdued but thoughtful man who knew what was coming and used the power of song to articulate his thoughts, make peace with his past, and wonder what was next.
''Unearthed'' wasn't conceived as a memorial; compiled before his Sept. 12 death, the set was intended as a 10th-anniversary commemoration of Cash's years with producer Rick Rubin, who revived the Man in Black's moribund career with a hip-cachet series of austere albums. Those discs, ''American Recordings'' (1994) through ''American IV: The Man Comes Around'' (2002), all on Rubin's label, recast the underappreciated icon as a seen-it-all interpreter of everything from folk ballads to alt-rock tunes. (The latter plan always threatened to turn Cash into the William Shatner of country, but, thankfully, rarely did.) ''Unearthed'' collects three discs of outtakes from those sessions, another that anthologizes the American albums, and a CD of recently cut spirituals.
In the liner notes, Rubin claims the two men experimented with many approaches before settling on the spartan sound of the American records. The most remarkable aspect of ''Unearthed'' is the way it allows listeners to feel like flies on the studio wall as Cash tries his hand -- and voice, with its well-deep bass notes and worn-leather intonation -- at anything he pleases (or Rubin requests). Cash summons up murder ballads and train songs, takes a crack at sturdy new tunes that seem to have been written for him (''A Singer of Songs''), and tests out spry new material of his own (''No Earthly Good,'' his miffed take on the Christian right). He also revisits songs he'd written decades before. When it was cut in 1970, ''Flesh and Blood'' was a blue-sky celebration of his love for his wife, June Carter Cash; tackling it again more than 20 years later, Cash sings like a man looking back, with pained gratitude, over times good and bad. It's also easy to see why he resurrected an even older original, ''The Caretaker'': When he sings ''Who's gonna cry/When old John dies,'' he's eulogizing himself as much as the character. Stark and maudlin for sure, but thanks to Cash's knotted-oak-tree voice and Rubin's hands-off technique, these tracks are also intimate and mesmerizing.
Simultaneously, ''Unearthed'' makes it clear that Rubin was right to surround Cash with as few instruments as possible. Although Tom Petty and the Heartbreakers may have loosened Cash up, their shelved collaborations with him are musically conventional. (The exception is a quasi-psychedelic take on Neil Young's already surreal ''Pocahontas.'') Cash and Rubin were smart to can versions of ''He Stopped Loving Her Today'' and ''Wichita Lineman,'' since they lacked the melodramatic grandness and poetic melancholy, respectively, of the George Jones and Glen Campbell originals. On the other hand, it's too bad they waited until now to release Cash's ruddy duet with Joe Strummer on Bob Marley's ''Redemption Song'' and two slices of bump-in-the-night roots rock with the Red Devils.
As for the rest, the disc of spirituals is a tad monotonous yet undeniably personal, especially those songs (like ''Never Grow Old'') that allowed a declining Cash to envision heaven. The best-of-the-Rubin-years CD wisely cherry-picks the most majestic moments (Goth-folk covers of Nick Cave's ''Mercy Seat'' and Nine Inch Nails' ''Hurt'') and omits the forgettable ones (a shaky ''Bridge Over Troubled Water'' with Fiona Apple). No matter the song or context, though, the recurring theme of this overwhelming and overstuffed postscript to Cash's Rubin years is the notion that taking solace and finding deep truths in music never end, even as death looms. As depressing as ''Unearthed'' can be, it's also one of the year's most oddly comforting releases.