Carkeek Park, filled with its forest of trees and brush, is one of the best places to get out of town. But the park that hides you from the city is also hiding a part of the city from you. A concentrated, repellant part, unmentionable in polite circles, locked behind a door and obscured by sword ferns. Unfiltered Ballard pools there, in all its pungent glory. The legend is that the nose never forgets the smell of raw sewage. It wasn't the kind of thing I wanted to know about. But I had to ask.

The question; what is the deal with that place? I'd asked myself that several times while looking at a compound at the bottom of the park. It's a fenced off area, sitting between a creek and a hillside, camouflaged by tall trees. In the compound sit a few pleasant, pink-brick buildings and what looks like a sunken pool with pipes protruding from it.

A fish hatchery, right? Salmon and what not?

"It treats combined sewer overflows," said Annie Kolb-Nelson, a community relations planner with King County's Waste Treatment Division (WTD).

"I see," I said, not understanding.

And that's really at the heart of sewage treatment; a willful non-understanding. A delicate, collective denial about what it takes to live among so much human density. We spend our whole lives thinking about what to eat but always drop the topic as it goes down our throats.

Some things are better left unsaid, but I kept on talking.

"Can I see it?" I asked.

Kolb-Nelson said it would be boring. Combined sewers only overflow in the winter, during heavy rains. Now it was the (for Seattle) dry summer. Nothing happening.

No problem, I said, in part because seeing how sewage treatment worked without seeing actual sewage seemed like a pleasant way to tackle the topic.

Kolb-Nelson and another WTD employee, water quality planner John Phillips, met me at the little facility, which was quiet and tidy, surrounded on all sides by greenery, and lacking in anything that might be construed as sewage. It seemed like a pleasant place to take lunch; munch Brie and drink Chardonnay and think in very, very, very abstract terms about "wastewater treatment influent".

The treatment plant's most obvious features are a couple of brick towers, which store grit and sand, substances invariably dragged into storm drains and filtered out of the system before they damage equipment. There are also a pair of buildings housing the pumps that provide the horsepower for the sewer system.

A pair of recessed tubs, called diversion structures, do the plant's dirty work, separating out solid sewage from the overflow. What's left is mixed first with bleach, then a bleach neutralizer before the sterilized water - "effluent" to WTD - is pumped about a quarter mile out into Puget Sound, through a pipe running under Carkeek's beach.

The diverted solid sewage, which can be anything you can get down your kitchen drain - as well as the usual suspects - is sent off to King County's West Point Plant, in Magnolia, for more sophisticated treatment.

Kolb-Nelson and Phillips explained all this with aplomb, never leaving a detail out, never a hem nor a haw. They answered every question; even the usual barrage of remedial, poorly prepared ones.

I thought I was coming to understand sewage treatment. But since nothing was being treated, the process had a pleasant glow to it; in my mind I saw trusty machines, automatically springing to life on rainy, winter days to capture whatever murky substance might constitute a combined sewer overflow.

Combined sewers are nothing more than sewers that hold both storm water and raw sewage in the same pipe. Sewage gets to the pipe from traditional household evacuation routes, storm water from storm drains. During heavy rains, water from the street can overwhelm the normal capacity of these pipes and without a release valve, the sewage, like some terrible salmon, would return to its spawning ground. Combined sewer overflows (CSOs) are the release valve.

Combined sewers are a legacy from days when sewage was simply dumped raw into the sea. As late as the 1950s, the good people of King County dumped 20 to 30 billion gallons of raw sewage into bodies of water around Puget Sound every year. In 1960, sewage treatment began and sewers were built routing human waste to those plants. In the 1970s, the county passed a bond measure - raising almost as much as a viaduct rebuild in today's dollars - for funding digging up the legacy of combined sewers and replacing them with separate lines for sewage and storm water. In 2005, only about 20 percent of the sewers in Seattle were combined and less than a billion gallons of storm-watery sewage was discharged untreated. If a billion still sounds like too big a number, the final hurdle might require billions more taxpayer dollars, digging up streets and eminent domain for homes in the way.

So we live with it, or at least don't think about it. But every year there's a chance that one of the 150 or so CSO locations that dot the ship canal, Lake Washington, the Duwamish River, Puget Sound and even Green Lake might dump raw sewage.

But Carkeek Park is different. Here, the rare overflow - four times in the 2004 2005 reporting period - gets treated by the plant. When the overflow comes, these dormant pumps kick in and push the wastewater through the treatment process. But then, some of the pumps aren't dormant.

In the last building I saw on my tour of the plant, there was a set of pumps howling away at earplugs-required levels. Was it a combined sewer overflow? In mid-summer?

No, it was just the Carkeek Park plant, doing its thing, like it does every day, in its primary role as a pumping station, pushing millions of gallons of sewage through pipes up from the Carkeek Basin, some 200 vertical feet to the next pump station on 8th Avenue Northwest.

Up to this point I hadn't asked where these overflows come from that the plant treats. They overflow at the wet well; the place where Carkeek the pumping station and Carkeek the treatment plant meet.

Can I see it, I asked?

"Well, ...it's where the rubber hits the road," Phillips said, laughing somewhat apologetically.

Then I laughed, but out of ignorance.

Sewers are all about pumps and pipes. Small, city-owned pipes get sewage from your home to larger pipes and then mains, which aggregate in still larger pipes. The largest pipes in the system are called interceptors, and they are managed by the county; the sewage highway to the city's county roads.

Such an interceptor runs under Carkeek Park. It runs into and then out of the wet well. The wet well is a 45-foot deep cavity of swirling, raw sewage, ready to rise up and spill over the top during a CSO event, like a bottle fizzing over, so pressure doesn't back up the pipes, and the sewage can be captured for treatment.

I stood over it, looking down through a grated floor into a briskly moving current, opaque, and foamy. It was like standing above a gigantic blender while someone whipped up a pork chop smoothie.

I almost had time to write "it wasn't so bad" in my notebook when my nose was finally able to grasp the concentrated truth that wafted all about me. The kind of smell that, once inhaled, corrupted any other smell you tried to recall in your own nasal defense.

A tickle started behind my ribs and moved up, clenching my throat, while my stomach tried to fold itself out of existence.

The only thing comparable to that sensation was a long ago memory, of once being violently seasick, during a storm on the English Channel. I threw up prawn sandwich all over a ferry boat bathroom stall then. Now, the Channel, the sandwich and the bathroom stall merged inside the wet well to form a rancid sensory overload; my eyes seeing the river, my ears hearing the gurgle, and my nose, begging my stubborn mouth to take over breathing. I had to steady myself against a handrail. It was sticky.

We all know that cops protect us from the city's dark underbelly. Well, there's a darker place under the underbelly and King County's Waste Treatment Division protects us from that. I know.

On my way out of the plant, Phillips intoned:

"They say the nose never forgets that smell."

I can still see him uttering that phrase like some mummy's curse. Even the memory is maligned now, so in my mind's eye, Phillips says it while tapping his nose and winking.

As far as I know, the nose never forgets the smell of raw sewage. But just take my word on it.