The visceral test of a claim like this is whether the numbers hold up under examination. On the available evidence, they do. The cliff is called Verona Rupes, it is located on the small icy moon Miranda in the Uranian system, and it is currently classified by NASA as the tallest known cliff anywhere in the Solar System.

The 20-kilometre figure is worth pausing on. The tallest building humans have ever constructed, the Burj Khalifa in Dubai, is approximately 830 metres from ground to spire. Verona Rupes is roughly 24 times taller than that. The Grand Canyon at its deepest point is approximately 1.8 kilometres from rim to river. Verona Rupes is roughly 11 times deeper than that. Mount Everest, from base camp to summit, involves approximately 3.5 kilometres of vertical elevation gain. Verona Rupes exceeds that by nearly a factor of six. The tallest vertical cliff on Earth, Mount Thor on Canada’s Baffin Island, is approximately 1,200 metres from base to top. Verona Rupes is roughly 17 times taller.

The moon

Miranda, on which Verona Rupes is located, is the innermost of the five major moons of Uranus. It is approximately 470 kilometres in diameter, roughly the size of a large European city if that city were spherical and made mostly of water ice. The other four major Uranian moons, Ariel, Umbriel, Titania and Oberon, are all substantially larger than Miranda, and along with Miranda they were all named after literary characters from the works of William Shakespeare and Alexander Pope.

Miranda itself is named for the daughter of the exiled sorcerer Prospero in Shakespeare’s play The Tempest. The naming convention was set by the International Astronomical Union in the nineteenth century, when the two largest moons, Titania and Oberon, were named by their discoverer William Herschel’s son John after characters in A Midsummer Night’s Dream. Every Uranian moon discovered since has been named after a character from Shakespeare or Pope. Verona Rupes, the cliff feature itself, was named in the same tradition after the Italian city of Verona, which is the setting for several Shakespeare plays including Romeo and Juliet.

How we know

Everything we currently know about Miranda’s surface comes from a single spacecraft flyby in January 1986. NASA’s Voyager 2 probe passed the Uranian system on 24 January 1986, and photographed Miranda from a distance of approximately 29,000 kilometres. The images the spacecraft returned during that brief encounter constitute the entirety of the high-resolution photographic record of Miranda that human civilisation possesses.

Voyager 2 imaged only the southern hemisphere of Miranda. The northern hemisphere was in shadow at the time of the flyby, and no spacecraft has returned to the Uranian system in the four decades since. Everything humans currently know about Verona Rupes, including the 20-kilometre depth estimate, comes from analysis of the Voyager 2 images. The northern hemisphere of Miranda remains, in a substantial and specific sense, the largest unimaged surface of any major body in the Solar System.

The name Verona Rupes was adopted by the International Astronomical Union in 1988, two years after the Voyager 2 flyby, when the astronomical community had finished the initial analysis of the images and named the major surface features Voyager had revealed.

The physics of the fall

Miranda’s surface gravity is approximately 0.008 times Earth’s, or approximately one one-hundred-and-twenty-fifth. The specific reason a person could conceivably survive a fall from Verona Rupes is that this gravitational difference produces a fall that behaves nothing like a fall of comparable distance on Earth would.

According to NASA’s Astronomy Picture of the Day analysis, published multiple times across 2007, 2011, 2016 and 2020, a fall from the top of Verona Rupes would take approximately 12 minutes to reach the bottom. The impact velocity at the end of that fall would be approximately 200 kilometres per hour, which is comparable to the speed of a racecar or a fast express train, and substantially less than the terminal velocity a skydiver reaches falling through Earth’s atmosphere.

NASA’s published assessment is that the fall might actually be survivable given proper airbag protection. The specific engineering problem is not comparable to surviving a fall from a comparable-height cliff on Earth, which would be immediately fatal for reasons of both impact and terminal velocity. On Miranda, the fall is slow, the impact is comparatively soft, and the atmospheric conditions are irrelevant because there is essentially no atmosphere to speak of.

The 12-minute duration of the fall is worth considering separately. It is long enough to read a chapter of a novel. It is long enough to have a substantial conversation. It is long enough, on the observable evidence of how the human brain processes extreme situations, to move through the standard sequence of emotional responses to imminent mortal peril, several times over, and to arrive at some kind of composed acceptance well before the bottom of the cliff arrives.

How it got there

The origin of Verona Rupes is still contested. There are two leading hypotheses in the planetary-science literature, and neither has been definitively established.

The first hypothesis is that Miranda was struck by a large impactor early in its history, in a collision energetic enough to disrupt the entire moon, and that Verona Rupes is the boundary between two large pieces of Miranda that reassembled from the fragments at substantially different heights. Under this model, the cliff is not the product of a single geological event but the accidental consequence of a shattered small body slowly gravitating back together with its pieces at different elevations.

The second hypothesis is that Verona Rupes was produced by tectonic surface motion on Miranda itself, at some point in the moon’s early history when it was warmer, more geologically active, and possessed enough internal heat to drive large-scale crustal rifting. Under this model, the cliff is analogous to some of the larger rift-valley features on Earth, produced by the same underlying physical processes, only at a substantially larger scale relative to the size of the body it is on.

The specific evidence that would distinguish between the two hypotheses, on the current state of the literature, requires imaging the northern hemisphere of Miranda that Voyager 2 could not see, and probably requires a dedicated Uranus orbiter mission to acquire. No such mission is currently under development or funded for launch this decade.

What it means

Verona Rupes is, on the current best measurements, taller than any comparable geological feature on any body humans have ever visited or imaged. It substantially exceeds Olympus Mons, the Solar System’s tallest volcano at approximately 21.9 kilometres, in terms of the specific vertical drop a person could experience while falling. It exceeds Valles Marineris, the Martian canyon system at approximately 10 kilometres deep, by a factor of two. It exceeds every mountain, every gorge, every canyon, every cliff, and every rift on the planet Earth by orders of magnitude.

It sits on a moon smaller than most European countries, orbiting a planet no human-made spacecraft has visited in four decades, at the edge of the region of the Solar System that current human civilisation has any direct information about at all.

The 20-kilometre figure carries uncertainty. The specific fall dynamics carry uncertainty. The origin remains contested. But the underlying claim, that the tallest known cliff in the Solar System is on a small moon of Uranus, and that a fall from its top would take longer than most commutes to work, and would be, at the bottom, only about as violent as a moderate car accident, is the current best interpretation of the available evidence.

Verona Rupes is the sort of place humans might, in some future century, actually go.

The fall would take twelve minutes.

Whether it kills you depends on what you’re wearing.