Workers at the Rossokha equipment graveyard cut up vehicles for scrap. Thousands of trucks, bulldozers, helicopters, and other vehicles used during the Chernobyl cleanup became so contaminated that they could no longer be used. Those that have had enough radiation washed away by years of rain and snow are melted down and the metal sold. The problem of how to cope with the still unknown human toll of Chernobyl is not so simply solved.

Not surprisingly, their new neighbors resented the torrent of evacuees and the preferential treatment they received as official Chernobyl victims. "Some people really hated us," says Olesya. "They said, we are living in a contaminated zone as well. But they got no compensation." Even today Ukraine and Belarus spend some 5 percent of their national budgets on Chernobyl, mostly on cash, holiday trips, and other benefits for the millions of registered victims.

Those who stayed behind still inhabit a contaminated landscape. The two most pervasive radionuclides from Chernobyl, cesium 137 and strontium 90, will remain in the environment for decades. Schools and other public buildings in southern Belarus are regularly washed down. Fields are fertilized with potassium to limit the uptake of cesium into crops and lime to block strontium. Lengthy regulations spell out what should be grown in which soils (only potatoes in peat but a wider range of crops in clayey soils, which lock up radionuclides). The most contaminated land—several hundred thousand acres—still lies fallow, though the government of Belarus is taking steps to reclaim it.

At a gate and guardhouse 18 miles (29 kilometers) from the reactor, cultivation stops entirely. Fields give way to forest, dark, fragrant, and still unsettling on my fourth visit in ten years to the exclusion zone, a tightly controlled area almost twice the size of Luxembourg. The first time, in 1995, a dog pranced up as I waited for guards to inspect my papers. I felt a stab of pity: A huge tumor had deformed its jaw. As the dog got closer, though, I realized that my anxiety about radioactivity had played a trick on me. The tumor was really a chunk of asphalt the dog was carrying in its mouth.

At the center of this accidental wilderness stands the sarcophagus, naval gray and malignant, and rustier than I remember. Built in six months, it was planned to last at most 20 years. One beam supporting the corrugated steel roof rests precariously on a severely damaged wall of the reactor hall, while the western side of the structure has bulged several inches. None of the joints were welded: Workers couldn't get close enough. Any of a number of freak scenarios—an earthquake, a tornado, a heavy snow—could bring it crashing down. Or the sarcophagus, also known as the shelter, could simply collapse on its own.

This fragile shelter holds an estimated 200 tons of nuclear fuel, some of it in the reactor core and some in an unearthly radioactive "lava"—fuel rods, concrete, and metal that melted together in the inferno and oozed into the warren of rooms beneath the reactor. There's enough enriched uranium and plutonium in the hulk for dozens of atomic bombs.

But the immediate threat is water. A few years ago workers measured more than a thousand square yards of cracks and holes in the sarcophagus, which were allowing rain and melted snow to pool in its bowels. The water further weakens the structure, and it seeps out into the environment, carrying radioactive contaminants. Water can also act as a nuclear moderator: a substance that aids a chain reaction. Though the risk is deemed minute, a renewed chain reaction could trigger another steam explosion, blowing open the sarcophagus, scattering chunks of fuel, and releasing tons of fine radioactive dust.

On the night of June 26, 1990, after two weeks of heavy rain, detectors in one lava-filled room registered a sharp rise in neutrons, a sign of an impending chain reaction. Four days later, a physicist from a technical center in the old town of Chernobyl, ten miles (16 kilometers) away, dashed in to pour neutron-quenching gadolinium nitrate on the lava. The neutrons subsided.

Similar selflessness over the years has taken a heavy toll. The technical center, run by the Ukrainian Academy of Sciences, is the home of the "stalkers," scientists who work in the sarcophagus, exposing themselves to high levels of radiation as they monitor the state of the reactor fuel. Near the entrance is a list of several dozen who have died, many in their 40s and 50s, many from cancer or heart disease. I recognize one, Edward Denisenko, whom I'd met a decade ago, and recall him musing about who in their right mind would want to work at Chernobyl. "If people from the West and Russia don't want to come here," he had asked, "who will? God? The devil?"

In the past two years 90 percent of the gaps have been plugged, and a new sprinkler system dispenses gadolinium in the central hall. Most rainwater is pumped out, though some is allowed to linger to suppress dust. But Yuliya Marusych, who works in the nuclear plant's information department, says flatly, "The shelter was and is risky. It's a threat to people working here, to the residents, and to the environment."

Marusych, a chain-smoker whose brown hair is dyed ginger red, takes me inside for a look. Radiation dosimeters in our pockets, masks on our faces, we pass through a series of corridors to a checkpoint where a plant engineer shows me a diagram marked with radioactivity levels. The hottest recorded spot in the sarcophagus, at 3,400 roentgens an hour, would deliver a lethal dose in a few minutes.

The deepest that Marusych can take me is the control room of reactor four. It was here 20 years ago that night-shift operators watched in horror as the chain reaction spiraled out of control. Though the ceiling tiles are gone, exposing pipes and a mass of wires and cables, the instrument boards are intact. Five years ago, the room was doused with a pink decontamination solution. Where the residue clings to the walls it looks disturbingly like blood.