The final third of Superterranean by Mimi Lien and Pig Iron Theatre Company is the loveliest, most exhilarating, saddest part of the show, running through Sunday at 2300 Arena in South Philly as part of the Fringe Festival. It’s ambitious and well worth seeing — but go with an open mind, a taste for experiment.

Lien is a MacArthur- and Tony-winning set designer (in 2017 for Natasha, Pierre & the Great Comet of 1812). In a big venue that also hosts boxing, wrestling, and MMA cards, she and Pig Iron take on a huge theme: our emotional encounters with the gigantic structures and spaces we have created in our urban landscapes.

Lien, eight actors, and the famed Pig Iron brain trust of lighting and sound designers, choreographers, movement coaches, and puppeteers are at their most original.

The first third features a split-screen effect, to stage left a cave-like space where dark, glittering figures huddle and crawl; and to stage right a tissue-y shaft down which descend a white balloon (it returns later!), a ragged chrysalis, and writhing, half-born entities. Naked and silhouetted, a woman (Mel Krodman) ponders this double-sided biological history.

Sound design (by Lea Bertucci and Toby Pettit) is spectacular throughout, from the thrumming electronica of the first two parts to the splashy textures of natural, electronic, and social sound in the third. In the second part, a machine with mist-emitting vents dominates a huge, bright room and the people in it. The room goes yellow, red, green, and the machine seems to change its voice, or voices, in different registers and rhythms.

Not a word is spoken in the whole production, but there’s plenty of action. Each person has his or her own uses and aspirations with the machine. One man kneels in supplication; a woman plants lips and hands on it, as if lovemaking; one guy (the splendid Dito van Reigersberg, far from Martha Graham Cracker) sticks his head way up the port.

One motif in the costumes (by Olivera Gajic) is the use of plastic see-through portals revealing stomach, chest, private region, or back, all sites linked to desire, drive, or suffering. A woman in a fur coat (again, the superb Krodman) seems to stand for agony itself.

Just as the second part is starting to seem claustrophobic, stuck, the walls fall away, revealing a whole city of machinelike structures. Bells ring, as if it’s time to be somewhere, and Rolls André, Evelyn Chen, Jenn Kidwell, Krodman, and company, purpose unknown, walk the misty landscape.

In this lonely communal ballet, van Reigersberg and Saori Tsukada circle one structure, caressing it with their hands and each other with their bodies. Chelsea Murphy, a determined long-distance runner much of the show, does a beautiful floor routine beneath a fluorescent light, and the rest follow suit. People gather, disperse, set lights swinging, abandon the stage. It’s gorgeous and feels like life.

Not everything works. But daring, transgressive Pig Iron is known for going big and scoring, and Superterranean continues that tradition. Lien and company have hit on a deep, familiar truth: Our self-made landscapes help shape our drives and emotions, our emptiness, ecstasy, and everything between.

THEATER REVIEW

Superterranean by Mimi Lien and Pig Iron Theatre Company

Pig Iron performances through Sept. 15 at 2300 Arena, 2300 S. Swanson St., a Fringe Festival show.

Tickets: $39–$45 ($15 for ages 25 and under)

Information: 215-413-1318, fringearts.com