When Elias Unger looked out from his front porch on May 31, 1889, he was astounded by what he saw. His house overlooked Pennsylvania’s Lake Conemaugh, a 2½ mile-long man-made body of water formed by one of the world’s largest earthen dams. On that morning, the rain-swollen lake was dangerously close to breaching the wall.

Gathering a work crew, Unger labored frantically to shore up the dam, but it was too late. By afternoon, the 72-foot wall had given way, sending 20 million tons of water surging down the Little Conemaugh River Valley toward Johnstown, Pa., and claiming the lives of 2,209 people.

Details: Johnstown, Pa.

On a recent visit, I stood near Unger’s porch, looking out over a pastoral valley dotted with trees and the Little Conemaugh gently meandering through it. A railroad track ran along the valley floor. I could see the remainder of the dam: two earthen abutments with a telltale 270-foot gap between them.

Unger’s farm is now the site of the National Park Service’s Johnstown Flood National Memorial in South Fork, Pa., about 14 miles upriver from Johnstown. Unger’s house is the original, but it’s not open to the public.

I’ve always been fascinated by the Johnstown Flood, so I’d come to the memorial to learn more about it. At the visitor center, I read about its causes: A mega storm in which about 10 inches of rain fell in the 24 hours leading up to the tragedy caused the long-neglected earthen dam holding back the lake waters to give way.

Photographs show piles of debris, a huge tree rammed through the front door of a house, freight cars tossed about. A display case shows Victorian mourning garb. The focal point is a display telling the story of Victor Heiser, a 16-year-old who survived the flood. A 92-year-old Heiser’s voice plays over speakers, while a model of him as a boy clinging to his barn roof hangs from the ceiling.

Park Ranger Doug Bosley explained that the lake was the site of the South Fork Fishing and Hunting Club, an exclusive private association whose members included the likes of Andrew Carnegie and Andrew Mellon. The rich and powerful of Pittsburgh built lavish “cottages” on the shores of the lake, some of which remain today, making it their private summer retreat for boating, fishing and picnicking. Club members apparently weren’t interested in maintaining the dam and actually lowered it so that carriages could cross the abutment. The people downstream would pay for the pleasures of the rich.

When the dam was breached, according to Bosley, “the water emptied out in about 45 minutes.” And in that interval a wall of water and gathering debris swept everything in its path — tree limbs, freight cars, houses and human bodies. So destructive was the force of the water that several towns in its path were leveled, and locomotives were tossed about like bath toys.

Standing on the dam, looking back at what was once the lake bed, I used my imagination to fill in the little valley with more than 70 feet of water. As I tried to picture the moment of the breach, I realized that this was just the beginning of the story; 400 feet lower and about 14 miles downstream, Johnstown was the end. I headed there to get a complete picture.

The Johnstown Flood Museum on Washington Street is housed in the former Cambria County Library, built at the site of an earlier library building that the flood destroyed. Andrew Carnegie donated the funds for the new library.

The massive French Gothic structure contains three floors of display cases, photographs and drawings that chillingly depict the flood and its aftermath. Flood relics include a chair, a pitcher, keys, a trunk, a pair of binoculars: the everyday items of life that were swept away in 10 awful minutes. One photograph shows a dazed family on the side of the road, another stacks of coffins, some heartbreakingly tiny. There’s even a vial containing floodwater.

According to volunteer docent Richard Hambright, Johnstown’s flood was the second-greatest natural disaster in the country’s history in terms of loss of life, the largest being the Galveston Hurricane of 1900. Hambright, a retired schoolteacher, led a group of us to a fiber-optic three-dimensional model of the flood’s path, offering a bird’s-eye view of the landscape and the settlements along the Little Conemaugh as they existed on that day.

At a signal from Hambright, the display case lit up and the sequence of the flood started. The water in the lake, represented by blue light, started to “move,” spilling out into the Little Conemaugh River. We stared transfixed as the light traveled downstream. Side panels describe what’s happening, and the sound of rain, rushing water, telegraph warnings, train whistles and people’s screams complete the experience.

Hambright told us that the worst part of the disaster was the fire that broke out at the Stone Bridge in Johnstown when a kerosene tanker caught fire amid the piled-up debris, trapping hapless flood survivors. “Eighty people trapped in barbed wire and debris burned to death,” Hambright said. “The fire burned for three days.”

An excellent film shown hourly at the museum explains the flood’s story in considerable detail. It relates so many side stories: a train engineer racing his engine backward, train whistle blaring, to warn the people of the approaching torrent; a train idling on the rails as passengers watch the wave approaching — some jumped to safety and clambered up the hillsides, while those who remained on board drowned; and the horrible fiery deaths at the Stone Bridge.

Rebuilding after the flood was one of the great triumphs to come out of the Johnstown disaster. Clara Barton and the American Red Cross arrived and helped the survivors for five months. Donations poured in from around the country.

The Pittsburgh Relief Commission purchased prefab housing to shelter homeless survivors. About 400 “Oklahoma houses,” so named because they were built for homesteaders headed to the just-opened Oklahoma Territory, were sent to Johnstown as temporary shelters. A surviving example sits next door to the museum; you can look through a glass partition at the four-square structure with its simple household furnishings. Think of it as an early FEMA trailer.

“They were emergency housing not suited to Johnstown winters,” said Richard Burkert, president and chief executive of the Johnstown Area Heritage Association (JAHA). “People would stay in them a year or two until they got back on their feet.”

In reality, it would take years for the once-thriving steel town of 30,000 to get back on its feet.

Before leaving town, I took a walking tour following a JAHA map that took me to the United Methodist Church on Franklin Street. It was one of the few buildings in Johnstown to survive the flood. Although it was severely damaged, this sturdy structure withstood the fury of the raging Little Conemaugh River, in effect parting the waters and shielding several buildings behind it from the destructive wave.

Looking at the church in the gathering twilight, I had a hard time imagining that during the flood, the water where I stood would have been more than five times deeper than my height, and that anything could survive it.

But the church is still there — and so is Johnstown.

Lee teaches journalism at Bucknell University.