The "slow one" is creeping forward from the San Andreas fault — and we don't mean an earthquake.

A muddy spring that had been dormant since it boiled up more than 60 years ago began a slow-moving trek across the land 11 years ago. Now, as it picks up speed — relative to its usual speed — it threatens a highway, a railroad line, an oil pipeline and telecommunications line in California's Imperial County.

And there doesn't seem to be a good way to stop it.

Disaster creep

Dubbed the Niland Geyser, this muddy spring that smells of rotten eggs first appeared in the 1950s, near the Salton Sea. It didn't move for decades, seemingly content to bubble away at its source. But within the past 10 years or so, it has been on the move.

The spring's movement has been slow to a certain degree, sometimes taking months to move 60 feet (18 meters). Recently, however, it has started to move at a faster pace, making its way forward 60 feet in a single day. In total, the pit has moved 240 feet in a decade, with its speed increasing since 2015.

"It's a slow-moving disaster," Alfredo Estrada, Imperial County’s fire chief and emergency services coordinator, told The Los Angeles Times.

The mud spring has a lot in common with sinkholes, at least in how they form. The movement of water and other liquids deep underground erodes minerals and rocks and forms cavities. The spring expands upwards from this point, until it breaches the surface and forms this pit on the ground while continuing to erode from below, U.S. Geological Survey geophysicist Ken Hudnut told The Times.

This is not a spring you want to take a mud bath in, however. About 80 degrees Fahrenheit (27 degrees Celsius), the spring's bubbles come not from soothing hot water but from carbon dioxide boiling up from the Earth's depths. The CO2 is likely the result of thousands of years worth loose sediment from the Colorado River being pushed deeper and deeper underground, Hudnut explained. That sediment is turned into CO2-emitting stones, like greenschist rock.

The San Andreas fault zone on a clear day with a deep blue sky A slow-moving muddy spring near the San Andreas fault area isn't a sign that the 'Big One' is coming. (Photo: Chris Geszvain/Shutterstock)

So between the lousy smell and the lack of oxygen, anyone unfortunate enough to fall into the spring would die within minutes. Thankfully, the CO2 eases up a few feet away from the spring.

The real threat is the spring's ability to simply consume land. Today, the spring has gotten close enough to Union Pacific railroad lines that connect the Inland Empire to Yuma, Arizona. Union Pacific has been working for months to halt the spread of the spring, draining water from it and constructing a 100-foot-long and 75-foot-deep wall of steel and boulders to protect its lines.

In October, the spring simply slid under the wall.

Union Pacific has constructed temporary tracks, but more permanent solutions may be necessary, including a bridge over the affected ground. Freight is already moving more slowly in this corridor as a result of the spring.

Highway 111 is also a potential victim of the mudspring's approach. Caltrans, the California Department of Transportation, has already planned a series of detours, a spokesperson for the agency told The Times.

Verizon-owned fiber optic lines and a petroleum pipeline owned by Kinder Morgan, one of North America's largest energy companies, are also in the spring's path.

The one bit of good news is that the spring isn't a sign of impending seismic activity. According to Hudnut, the areas has been relatively quiet for months.

Small solace to the railroads and highway systems.

A 'slow moving disaster' has bubbled up from the San Andreas fault
A bubbly mud spring called the Niland Geyser is threatening a railroad line and highway in Imperial County, California, with its slow creep.