Looking for something to do in Kansas on a sunny Saturday last spring, I chose darkness, and drove to Hutchinson (pop. 42,000) to take a tour of the subterranean salt mines at the edge of town. At a museum, you buy a ticket and ride a large, clanky freight elevator 650 feet underground, into pitch black.

There was no crowd. I had the vast, dimly lit caverns almost to myself — and what a place. A long series of featureless gray tunnels gave up millions of tons of rock salt over the last century (and still do), excavated over decades by laborious room-and-pillar mining techniques. Down there, it’s always dry and a perfect 67 degrees. You take a little train through the dark and learn a lot about drilling and geology.

The most interesting part of the mines, however, is entirely off limits. Nearly 2 million square feet of tunneled rooms have been profitably repurposed for film and document storage. Hollywood rents some of the space and so does the government. Shelf after shelf of original negatives of movies and old TV shows are kept here below the prairie, safe from almost any apocalyptic or meteorological scenario you can imagine, secure even from the hungry eyes of YouTube and Hulu.

Included in this stash are more than 4,000 episodes of the legendary “Tonight Show Starring Johnny Carson.” His farewell episode aired 20 years ago this month. At some point, the entire archive of tapes — the rights to which Carson fully acquired in one of his many highly publicized contract renegotiations with NBC — came here for eternal rest. Except for the standard clips you see from time to time (one of Joan Embery’s small zoo critters urinating on Johnny’s head; various Carnac the Magnificent routines) and the retrospectiveDVD sets that come and go, the real breadth and achievement of the Carson era is difficult to grasp if you didn’t live through it.

“Johnny Carson: King of Late Night,” a heartfelt and at times beautiful and melancholy two-hour documentary airing Monday night on PBS’s “American Masters” series, is a fine effort by filmmaker Peter Jones to both exalt and deconstruct the man. He has access to all the great stuff down in the salt-mine vault, but he’s after something deeper in Carson’s private persona. Talk about dark caverns. What we come away with is a portrait of someone who was brilliantly witty and easygoing, so long as the camera was on.

When it wasn’t, well . . . people still aren’t quite sure of what to say. He was, by nearly all accounts, socially awkward. He was a straying husband, a sometimes tyrannical boss, an ambivalent friend, a remote father, and, for much of his career, a mean drunk. (His discussion of his drinking issues, during a famous “60 Minutes” profile by Mike Wallace in 1979, came at a time when such celebrity admissions were still news.)

We are asked to consider all these things on balance with the soothingly ebullient happiness that “The Tonight Show,” under Carson’s stewardship, brought bedtime viewers for close to 30 years. The joy comes out far ahead, but a bittersweet aftertaste remains. Carson is still a tough nut to crack.

Going to the same thematic places where most Carson biographers have already trod — they include Laurence Leamer, Bill Zehme and Bill Carter; each interviewed here — Jones’s film nevertheless shapes a penetrating and refreshingly frank look at the man who mapped out the turf that is still so ardently fought for in the seemingly endless “late-night wars.” Most of the current warriors are interviewed anew in “Johnny Carson: King of Late Night” — Conan O’Brien, David Letterman, Jay Leno, Jimmy Fallon — and they provide typically reverent homage.

That’s often a problem in documentaries about showbiz — for every half-moment of insight and unguarded honesty, there are many more minutes of requisite Hollywood fawning. The film takes the long route to the not-so-new fact that Carson could be a jerk. Many of Carson’s comedian friends are interviewed, from his peer group as well as those to whom he bestowed big breaks: Mel Brooks, Dick Cavett, Don Rickles, Carl Reiner, Bob Newhart; Jerry Seinfeld, Ellen DeGeneres, Ray Romano, Garry Shandling, Drew Carey. Nearly all of them say what you expect them to say, along the lines of what they all said in tribute in January 2005, when Carson died at 79.

Good ol’ Joan Rivers, who will usually and fearlessly say anything about anyone, takes a whack at reassessing the public falling-out she and Carson had in 1986, when she signed a deal to host a late-night show for the then-fledgling Fox network. Rivers, a frequent “Tonight Show” guest who had risen to the permanent guest-host spot, neglected to tell Carson of her Fox deal and that was that, she says: He never spoke to her again. Twenty-five years on, she seems to regret what happened only a teeny, tiny bit; for her, that story is more about Carson’s capacity for anger in those waning years of his rule.

Unless the venue is a Friars Club roast, comedians are lousy interviews when it comes to the subject of one another — or departed legends. They fall back on safe anecdotes, temporarily surrendering their powers of devastating analysis. Documentarians still haven’t figured out that there is hardly anything more boring than a bunch of comedians talking seriously about comedy. (I have one exception to that pronouncement: The 2005 documentary “The Aristocrats,” which was dedicated to Carson, and explored the undercurrent of self-loathing that haunts most comedians and thereby elevates their craft.)

Just as it seems Jones’s interviewees are simply going through their routine memories, the film is momentarily rescued by Drew Carey, who becomes verklempt when trying to express his awe at, and undying gratitude for, his first appearance on Carson’s show, in 1991. After Carey finished his stand-up set, Carson beckoned him to the seat next to his desk — an infrequently bestowed rite that all comedians on “The Tonight Show” sought but rarely got. Unless I missed something (besides some tender reminisces from former “Tonight” bandleader Doc Severinsen, now 84), Carey’s are the film’s only freshly shed tears.

For heartache, “Johnny Carson: King of Late Night” is best when ruminating on Carson’s early years — way before “Tonight” — using old photos and movies of his Nebraska boyhood and nascent career. Carson grew up with one of those humble Midwestern upbringings people of his generation talk so much about: the work ethic, the adherence to the status quo, the covered-up emotions. “Apparently it was not the warmest of families,” Cavett intones.

No kidding. In a family that rarely articulated its feelings (love, actually), young John found refuge in magic tricks and performing onstage. An old how-to book, “Modern Magic” by one Professor Hoffman, is offered as Carson’s “Rosebud” object — stirring his almost lifelong dependency on the sound of applause. Only late in life, cleaning out his deceased mother’s closet, did he discover a box of tenderly clipped newspaper articles that chronicled his rise to TV superstardom. He kept it as the only real proof of her affection and approval.

After his wartime stint in the Navy and a degree from the University of Nebraska in 1949, the dashing young Carson went into radio but wound up on primordial television, on an Omaha morning show called “The Squirrel’s Nest.” Soon he packed up his family (his first wife, Jody; their three sons) and moved to Los Angeles, where, from 1951 to 1954, he hosted a local comedy show called “Carson’s Cellar.” Then Red Skelton called Carson and asked him to . . .

Wait, do you know all this?

Many of us do; some of us don’t.

Television is undergoing such radical and constant transformation that the old guard feels duly obligated (why, exactly?) to keep reveling in its history. Hence all the multi-part documentaries about TV’s golden era; all the broadcasting museum exhibits and retrospectives; all the clip jobs whenever an anniversary looms or a legend dies. “Johnny Carson: King of Late Night” assiduously steers away from too many sentimental lapses. There is a sense of thematic control over the material.

That’s good, because there is so, so much material down there in those old caves. Jones, whose previous documentary subjects include Ozzie and Harriet Nelson, Samuel Goldwyn, Bette Davis and the powerful Chandler family of Los Angeles, whittles it down with purpose and precision.

Carson took over hosting “Tonight” from Jack Paar in 1962, with the equivalent of the hyperventilating media ramp-up, speculation and anxiety that still accompanies such transitions. Carson’s version of the show caught on, and we see “The Tonight Show” slowly but surely discover its place in American cultural history.Ed McMahon’s permanent role as sidekick is also examined (with fresh insight from McMahon’s daughter, Claudia). The friendship and working relationship between the two men grew more complicated as the seasons went on.

Color comes along, with the ferns and the garish backdrops. The ever-widening lapels, the lengthening sideburns, the Megan Draper-esque second and third wives. There were a total of four; Wife No. 2, Joanne, who won a reported $100,000-a-year alimony payment for the rest of her life, is the only one interviewed here, and her memories are largely fond. Carson made great monologue fodder from his costly and protracted divorce settlements — the documentary treats all of this as subtext to the private angst, gently poking sources to see what else they observed about all that. (Neither of Carson’s surviving sons are interviewed in the film; but his nephew, Jeff Sotzing, who also worked as a “Tonight Show” producer, is.)

This is the cad Carson I first remember the adults chortling over in the early and mid-1970s. There was something about him that quite clearly appealed to our mothers; something slightly less definable lured our fathers, but surely it had to do with the fact that any one of them would have loved to have been as quick and sharp as Johnny — to say nothing of the pretty new wife (whichever), the yacht, the Malibu house, the DeLorean, the year-round tan.

I only ever got to stay up and watch “The Tonight Show” on motel TVs during family road trips in the summer. Something about his show must have worn thin for my parents by 1980 or so, when they switched to “M*A*S*H” reruns after the news. By the time I watched Carson myself, in the ’80s, it all seemed ancient and preserved — leftover remnants of someone else’s heyday. All I wanted was for it to hurry up so I could watch NBC’s “Late Night With David Letterman,” which had everything a college kid could desire: Pee-wee Herman; Larry “Bud” Melman with his toast on a stick; Stupid Pet Tricks. This isn’t meant to place me (or you) on a timeline. It just means that everyone thinks of a different gold when the subject is the “golden era” of late night.

Jones’s film continues balancing Carson’s happiness with that inescapably downbeat vibe. Retirement seemed to bring him real peace; personal awkwardness dogged him to the end. Save for an appearance on Letterman’s CBS show, Carson’s highly rated farewell in 1992 was pretty much the last TV audiences saw of him. “Johnny Carson: King of Late Night” may instill in viewers a true wish to have Carson back on regularly, in nightly reruns.

And although the documentary doesn’t explore the present-day moonscape of late-night programming, one can’t help but take the opportunity. Seeing Carson again, even in old clips, is a reminder of where we stand with the current lineup of hosts — a subject that still preoccupies far too much press and analysis.

What we have, in the combined form of all our late-night hosts, are competing shards of the greatness everyone ascribes to Carson alone: Some nights Letterman is a shell; other nights he shines. Some never miss Jon Stewart and Stephen Colbert — and never miss an opportunity to recap the best bits for you, smugly. You can develop a Jimmy Kimmel habit for a while, until you tire of him. Then it’s off to Craig Ferguson’s monologue, which runs the gamut, and is often great, with some of Carson’s assured manliness. Jimmy Fallon, who seemed so inept at this sort of thing a few years ago when he took over NBC’s “Late Night” slot, now excels at it with easy insouciance. And Conan, a highly paid refu­gee in­ cable’s outer camp, is still occasionally picking at the permanent Leno scar. (And the less said about Leno, who still and perhaps forever occupies Carson’s spot, the better.)

They all get clipped and posted the next morning for the daily online diet of “did-you-see?” tweets, blog items, entertainment sites and gabbing on social networks — a system that works marvelously for those of us who are known to drift off before 11:30 p.m. The sense of a full show (monologue, sketch, guest, another guest, barely time for the musical guest) is another lost concept in the race for fast attention.

“Johnny Carson: King of Late Night” keeps returning to Carson’s boyhood fixation on magic and finds it conclusive. When we talk about the pinnacle of late-night talk shows, we are talking about something illusive and momentary, even if lasted 29 years. You blink and it’s vanished.

American Masters: Johnny Carson: King of Late Night

(two hours) airs Monday at 9 p.m. on WETA and MPT.