By definition, fiction, even fiction based on or inspired by reality, is unreal, is posited, isn’t definite. The same goes for science fiction, which can be usefully described as speculative, simultaneously exploiting the imaginary and the rational. Yet all of “sci-fi” is informed by nonfictional elements, such as journalism, autobiography, history, psychology, sociology, or the hard sciences. The impulse to go beyond what’s definitely known or experienced can be a method of escapism—some entertainment functions this way; so do some iterations of religion; so does fear—but it can also epitomize curiosity, empathy, humility—humanity at its most virtuous.

You could provocatively, oxymoronically think of Containment and The Visit—two films that screened at this year’s expansively programmed Sheffield Doc/Fest, the latter as part of the festival’s Ideas and Science sidebar—as documentary science fiction, since both are concerned with speculated events rather than recorded ones. But in different ways, they both definitively document the fact of our collective uncertainty about ourselves, our planet, our purpose and our future. What we do to each other and our environment has real-world effects, and analyzing those effects can lead to informed speculation. And the motions to speculate, the emotional impulses toward speculation, are what these two works of nonfictional film invaluably and movingly explore.

“It’s so difficult to conceive of anything that is unknown,” says one of the experts toward the end of Michael Madsen’s The Visit, and the entire film is both a demonstration of that difficulty and a testament to the power and beauty of the attempt. Madsen doesn’t try to blur any lines between fiction and nonfiction, between what’s known and what isn’t—he instead makes it all meticulously clear, and instead searches for a language to mine the gap between the two. The central conceit of The Visit is that we, the viewers, represent an alien force arrived on earth. “The scenario begins with the arrival. Your arrival,” Madsen says via voiceover, as the camera tracks past pedestrians in slow motion. “Welcome to our planet.” We are invisible, unknown and theoretical to the experts, scientists, astronomers, psychologists, lawyers and government officials featured on camera—even more than we, the audience, normally are to those involved with making movies—and yet we are their subject, their concern. Which means they don’t merely directly address us by looking and speaking into the camera—a familiar, even tired documentary ploy—they also enquire of us, speculate about us, worry over us. The entirety of the film is pitched to an audience about whom nothing can be assumed, and to whom everything must be explained.

From the outset, this makes for a surprisingly, unsettlingly moving experience. There’s a strong formal intent behind the setup, but in practice it also involves treating the audience with more respect and reverence than we ever would be otherwise. “I’m really interested to learn, how do you think?” asks an expert. “I don’t mean to be nosy, but how is your mind constructed and composed?” Their questions can be simple, honest and overwhelmingly tender. Madsen shoots them dead on and dead center in the frame, which comes across less as Wes Andersonian stylization than focused, controlled, universalizing. “I wonder if there’s something that you see about humans that we don’t see ourselves,” asks another. It’s hard to convey the effect of being asked these questions, of being the unknown, of being worth knowing. Another questioner already anticipates the futility of the exchange. “If you are truly alien, will we ever understand you?”

Rather than an exploration of what aliens might look or act like, The Visit becomes a supposition of how a people and their planet might look to aliens, and how the mere fact of an alien visitation is likely to affect and provoke those people. And what’s shown, outside of the direct address interviews, is not shots of space, or animated fictions, but footage of Earth. The camera floats down city streets and over grasslands, through gardens and into buildings, utilizing steadicams, cranes, helicopters, slow motion and jump-cuts—anything to foster strange sensations, new perspectives. They’re familiar images, seen as if for the first time.

After those initial questions of us, the insecurities engendered by an alien visitation slowly start to come to the fore. An attorney starts sketching out an intergalactic legal agreement with us, mentioning property, self-protection, and justice, and wonders about a moral obligation between one another to help in times of need. A biochemist in a haz-mat suit genially swabs for tissue samples, then another expert surmises that we should be quarantined from the population in case our biochemistry is fully foreign from the world’s and prone to inadvertent, end-game, cellular competition. “Fear can be good in that it can lead to precautions,” an expert says, “but it can also take us past what’s sensible.” Two retired officials from the British government appear anxious and flummoxed in the face of our mysterious presence, preparing a public statement that carefully tries to anticipate and manage the panic it will inevitably cause. “People like to know what is happening, and if they can’t describe it, it’s normal for people to panic,” one says. “It’s a reactive situation.” There’s seemingly no getting around the fact that in being an unknown, we are a threat. “Uncertainty is probably the worst thing, politically, that can happen,” we’re told while watching men in fatigues prepare for engagement. At first a floating, observing eye, we’ve become something to protect against, an undefined monster that pedestrians flee from. The imagery evokes countless science fiction narratives, but this time we’re not only the monster being feared, we understand the base, purely impulsive anxiety that informs that fear. We’ve yet to say or do anything but show up—humanity takes care of the rest.

One of the most sensitive subjects turns out to be Doug Vakoch, who works at the SETI Institute as Director of Interstellar Message Composition. Tasked to speculatively communicate with imagined entities such as ourselves, he excels in the film at translating human tendencies and failings into heartbreakingly simple turns of phrase. Considering what man has done to less developed people for whom he has functioned as an alien force, one can only hope that visitors from space are more generous. “We’re hoping that in addition to being more technologically advanced,” Vakoch says, accounting for our interplanetary passage, “that you’re also more evolved morally than we are.”

Vakoch tells about the Voyager spacecraft that was launched into space in 1977 with the intent of informing potential intelligent life of our existence and culture. A de facto time capsule floating through and beyond the galaxy, Voyager was equipped with a copper phonograph record of music of various modes and cultures, and photographs of Earth’s natural and man-made beauties. “But do we talk about parts of ourselves we’re not very proud of?” Vakoch says. “Do we tell about war? About our ability to destroy our own civilization?” In the end, those realities weren’t mentioned in the Voyager materials, a fact also noted in Containment, Rob Moss and Peter Galison’s sobering look at the dangers that nuclear waste pose for future generations of humans and/or alien visitors.

While The Visit makes clear that humankind’s taste for war brings the threat of conflict to any encounter with the unknown, Containment is more concerned with the fallout of that warmongering, particularly the radioactive materials that we’ve yet to find a truly safe, let alone a fail-safe, way of disposing. Via the Voyager, mankind told “extraterrestrials the best about ourselves,” says a commenter, while warning against nuclear waste “requires telling our descendents the worst about ourselves.” Moss and Galison traverse the globe to prove that we are currently in more active danger than we would want to admit—they visit the site and surrounding area of the Fukushima reactor, the radioactive alligators and turtles of the Savannah River, and a massive, potentially unstable storage facility in New Mexico—but the film is most evocative when focused on far future disaster prevention.

With a plutonium half-life of more than 10,000 years, there’s little reason to expect that even the most securely ferreted-away nuclear waste is safe from exposure. Which begs questions about when, how and by whom the waste might be accessed. Myriad possible answers demand myriad tactics to employ, with nothing short of the fate of the world hanging in the balance. The film recounts various markers proposed and devised in the early 1990s by and for WIPP, the Waste Isolation Pilot Plant in Carlsbad, New Mexico. These include physical markers, such as pillars and barriers, and fields of spiky obelisks designed by the late architect and professor Michael Brill. (These markers are eerily, movingly mirrored by stone monuments left by a previous generation in Japan, warning against habitation below a certain land elevation because of devastating floods caused by tsunami.)

Supposing that physical markers erode, are moved, or even evolve in meaning over the millennia, speculators also worked on more organic, cultural methods of communicating danger. This led to a proposed museum and amusement park called Nickey Nuke and WIPP Worlds, which would incorporate a Mickey Mouse-like cartoon character and folktale about the dangers of the site, ideally passing down from generation to generation that the waste containers, the long, long legacy of our era’s folly, the Pandora’s Box of human insanity, are never to be opened. Speculating about how to secure the waste millennia into the future also inspired WIPP planners to think up possible scenarios in which the waste would become less secure, such as a super tunnel dug between Houston and Los Angeles, global illiteracy, a society of robots, an alien invasion, and a feminist revolution in which the laws and warnings of men are disregarded. The film visualizes some of these through animation that in turn suggest the style and iconography of science fiction. While largely more straightforward in its presentation of information and reportage, in these sequences Containment shows how the impulse to speculate—regardless of the reason or urgency behind it—inevitably leads to the fantastical.

It’s hard to fathom a terrestrial matter more abstract than how to communicate with whatever sentient being stumbles upon a storage facility currently buried thousands of feet below the ground tens of thousands of years in the future, yet it’s not surprising that solving for it led us into the realms of art and culture. It’s through art that we can imagine lives, times, and worlds beyond ours. Speculating is creating. In The Visit, the only overtly fictionalized sequence in the film involves a man in a haz-mat suit visiting the alien spaceship—which turns out to not be a spaceship at all, but rather a disorienting tour through a terrain of exclusively earthly locations. The visitor is played, or at least voiced, by Chris Welch, Space Engineer and Professor at the International Space University in France. At first it seems that his narration is coming from inside the suit, via fuzzed-out, Neil Armstrong-style walkie-talkie, but then, mid-sentence, Madsen cuts to Welch in a chair with his eyes closed, talking through the details of his imaginary journey. “The moment we think it possible,” Vakoch says, “reality expands.”

When The Visit ends, as it must, without the aliens having provided any of the answers asked of us, or having given cause to any military intervention, what remains are simple emotions that speak of the irresolvable human condition. Insecurity: “Why? Didn’t they like us?” someone asks. Sadness: another talks of “a collective depression.” Loneliness: “We are again alone,” someone says. “The normal condition of humankind.” And stepping away from the conceit, another says, “The probability that others exist is not the same as knowing we’re not alone.”

But in the craving for company, in surmising that we may not be alone, that we’ve been building to something greater than ourselves, we can imagine greatly. Where there is possibility we can create. And as Containment illustrates, that exemplary quality of invention, of speculating ways to make things better, can be marshaled toward solving for our very worst qualities. And as The Visit demonstrates through its own exquisite construction, we’ve no more powerful tool than our desire to know and be known, to love and feel love, to discover and be discovered. It’s what sends us deeper into the darkness, and further into the future, hopeful that we won’t be alone when we get there.