LEWIS COUNTY, Wash. -- Just outside of Chehalis is a geological wonder that you can't see.

Underneath your feet is a natural gas reservoir that supplies heat to more than a million Puget Sound homes on a cold winter day.

"It's unique in the state of Washington, nothing else like it we've discovered today," said manager Jim Janson.

What's above ground is impressive as well -- a maze of pipes 15 miles long and wells spread out over 3,200 acres. They're all powered by compressors the size of jet engines.

But what's unique about Jackson Prairie lies 1,800 feet underground. It's there that geologists who were looking for oil back in the 1950s stumbled across a thick, porous layer of sand saturated with water. It was no oil, but it still made for an amazing discovery since sand makes a perfect reservoir.

Roughly 40 million years ago, the sand was deposited there in what was then the Pacific Ocean. But 40 years ago, seawater was pumped out and replaced with gas.

All of our natural gas comes from Canada. In the summer when prices are low, Puget Sound Energy buys a surplus of methane and stores it in the reservoir to be used during the peak winter months.

To meet the growth of Western Washington, PSE recently expanded the reservoir.

"We've grown by three times how much gas we can take out on a peak day," Janson said.

The underground reservoir now covers a little more than a million homes.

The recent expansion cost about $44 million. The upgrade now makes it possible to draw more gas out of the ground faster, just in time for winter when gas is used at 16 times the normal rate.