Tatiana Khanlian is 2 years old. Over four months this winter, she had rashes along her arms, spontaneous bloody noses, mood swings, and a decreased appetite, her father says. Tatiana’s older brothers, ages 5 and 8, are old enough to know they couldn’t go outside, but not old enough to understand why.

“Imagine keeping kids away from playing outside,” their father, Gabriel, said.

It started in October. A pungent, sulfury smell, not unlike rotten eggs, hung over the neighborhood. By January, even when just whiffs of the stuff were floating through the neighborhood, it was still enough to cause headaches and watery eyes in newcomers. People who had been exposed for months were even more sensitive.

This is Porter Ranch, California, where the Khanlians are just one of the thousands of families that were evacuated from their home after a rupture in a massive natural gas storage reservoir launched the nation’s largest methane leak. The gas, treated with an odorant called mercaptan, is blamed for a wide range of health effects on residents in the Porter Ranch neighborhood of Los Angeles. In all, about a third of Porter Ranch’s families temporarily relocated.

Experts say health impacts are only temporary. According to the CDC, exposure to mercaptan can cause short-term headaches, nausea, weakness, fatigue, incoordination, and irritation of mucous membranes, but doesn’t report any known long-term effects.

But the scariest part might not be the headaches, dizziness, difficulty breathing, and irritated eyes. It’s that these illnesses are signs that the system failed to protect this small neighborhood in Los Angeles. In a state that uses 2.34 trillion cubic feet of natural gas every year, the system is everywhere — and nowhere more so than Porter Ranch.

Underneath 3,900 acres of hilly terrain overlooking the neighborhood is an almost unimaginable amount of natural gas. Originally an oil drilling site, the 115 converted wells that make up the Aliso Canyon Storage Facility can hold more than 85 billion cubic feet of natural gas. Southern California Gas Company (SoCalGas), which owns the facility, says the gas is used by the company’s 21 million customers in Southern California. It is piped directly to homes for heating and gas ranges, and it is sold to power plants for electricity. And according to the gas company, it is entirely necessary.

“Without Aliso Canyon, there might not be enough gas to supply customers during peak needs,” company representative Glenn LaFevers said during a public meeting in January. Then this week, officials warned that Southern California might face blackouts this summer due to the “significant risk” of natural gas shortages.

The facility is something of a clearinghouse for natural gas, some experts say — although data is notoriously difficult to attain, even for the state legislature’s oversight committee. It’s possible that all the natural gas used in Southern California goes through this one facility, officials have been told.

Lack of information is a theme here. Hardly any residents knew what the land was used for before the leak started. And they are hardly alone. Most of us don’t think about what generates the electricity that powers our computers and light bulbs, but hidden under mountains, beside schools, and beneath roads throughout the country is a vast network of natural gas storage and pipelines. The gas is stored in old oil wells and in aquifers, underground vacuums trapped in bedrock and invisible from above, save for a few nondescript metal well tops, hidden behind fences.