As PG&E takes the unprecedented step of turning off power to 800,000 customers in California, many residents are wondering: Why hasn't PG&E moved its power lines underground?

Doing so would stop lines from sparking wildfires, the cause of multiple destructive fires in recent years. But there are a number of reasons PG&E hasn't done much undergrounding — and most have to do with money.

It's a process PG&E has previously described as prohibitively (or nearly prohibitively) expensive.

As a 2017 San Francisco Chronicle story notes, it costs about $1.16 million per mile to install underground distribution lines. In cities, that number is much higher; work in San Jose cost $4.6 million per mile. Overhead lines cost about $448,800 per mile in comparison.

Most of the higher costs are associated with digging trenches for the lines. There is also the cost of repaving roads and other environmental factors, such as flooding or earthquakes.

"The cost of undergrounding a distribution line can vary depending on several variables, such as road width (work access), nearby sidewalks (to comply with the Americans with Disabilities Act), density of nearby residences and businesses, surrounding vegetation, the number of power lines involved, other existing structures underground and other environmental issues," PG&E writes on its site.

One place that will be getting underground lines is the town of Paradise, destroyed in the 2018 Camp Fire. The process will take five years, according to PG&E.

"Engineering an underground system requires designing the system around existing water, natural gas and drainage systems, as well as planning for future road widening and the possibility of a sewer system for Paradise," the utility announced in a May press release.

Despite the astronomical price tag, critics say money shouldn't matter when safety and lives are on the line. There's also a lot of bad blood built up between the utility company and Californians: In the aftermath of the 2010 San Bruno pipeline explosion that killed eight, news broke that PG&E diverted over $100 million in safety money for other purposes, including bonuses for executives.

In August, a judge denied PG&E's request to distribute $16 million in bonuses to 12 top executives. PG&E argued the bonuses would help incentivize executives to meet safety goals in the wake of multiple wildfires.

"There is simply no justification for diverting additional ... funds to incentive them to do what they should already be doing," Bankruptcy Judge Dennis Montali wrote at the time.

FULL PG&E SHUTOFF COVERAGE:

- Map shows Bay Area neighborhoods that will be without power

- PG&E map down? Here's an interactive NorCal power shutoff map that actually works

- Essentials to buy for PG&E's planned power outage

- Here's what you should (and shouldn't) do during a power outage

- When will my power turn off?

- Even more coverage here

Katie Dowd is an SFGATE Senior Digital Manager. Email: [email protected] | Twitter: @katiedowd