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The facility, owned by Southern California Gas Co., was the site of the largest methane leak in U.S. history in 2015, forcing more than 8,000 families from their homes.

"Without vents, methane and other gases remain trapped at the bottom, further multiplying the fragility of the loosely held structure."

But they are equipped with sensors that reveal the extent of methane, turning this invisible gas into concrete numbers on a screen.

In that time, CalRecycle inspectors also identified violations for elevated levels of flammable methane, improper signage and failure to conduct sufficient truck inspections to ensure hazardous wastes aren’t disposed at the site.

And although landfill operators routinely monitor for potentially dangerous gases, such as methane or sulfur dioxide, they typically don’t have instruments that would detect toxic contaminants in wildfire ash, like lead or asbestos.