The Pioneer Square Underground Tour tells most of us all we know about what lies underneath Seattle. It's a touristy exploration of the city's oldest basements, a jokey excursion through an urban underbelly filled with commentary about the foibles, and drinking habits, of the city's lusty pioneers. It's the closest many of us come to seeing firsthand the archaeology of the city.

You pass under sidewalks and see where old storefronts used to be before the streets were raised. You see the layers of cobbles, bricks, beams, and concrete that hold up our historic buildings. You learn how Seattle was rebuilt after the fire of 1889, how filling in the tideflats, re-grading the hills, and building the new on top of the old started us down the path where we are today. One thing certain: post-fire Seattle was not a city slowed by civic gridlock, nor daunted by re-engineering the landscape.

Once, years ago, the Underground Tour billed itself as taking visitors to see ruins larger than Pompeii's. If your archaeological expectations are set there, you'll likely be disappointed. Nevertheless, the tour gives you an idea of how complicated and layered the past can get, even while it forms the literal foundation of today.

Urban archaeology is complicated because of this layering of living city upon the buried iterations that came before. This isn't landscape where one can dig easily, or without economic consequence. But occasionally we do get a peek at what's under our feet, below the basements, sewers, utility lines, and shorelines. Often it's due to major public projects, like Metro's West Point Treatment Plant where in 1992 archaeologists found evidence of 4,000 years of habitation and use by native peoples. Archaeological finds have turned up during many Seattle projects, including those of Sound Transit, the Port of Seattle, and the World Trade Center, to name a few.

Such projects are required by federal and state laws to assess their impact on cultural resources, which often means sites of historic and archaeological significance. Such laws did not inhibit our ancestors, who built canals, highways and washed away hillsides will nary a thought for heritage, let alone the environment as we think of it today. But the National Historic Preservation Act (pdf) and the National Environmental Policy Act now lay out a process for taking such impacts into account, and taking them seriously. Failure to do so can have huge financial, political, and cultural consequences.

Seattle currently has two complicated multi-billion-dollar state mega-projects underway that have to be careful of what they do and where they do it. Archaeologists and cultural resource specialists are actively working to figure out what's at risk, and what opportunities the projects might offer for learning about Seattle's early history. The focus of this two-part series are the archaeological opportunities and challenges.

Even before major construction is underway, clues about what might be underground or under water are being assessed, and local tribes are being consulted. The archaeological sites are by no means only connected to Native Americans but include more than a century of habitation covering Seattle's birth, settlement, and emergence as a city.

The two projects are the Alaskan Way Viaduct Replacement project and the highway 520 bridge replacement at Montlake. Even though major components of each project remain undefined or debated (bridge and tunnel financing, the final design of 520 through Montlake, possible lawsuits,etc.), some archaeological impacts and possibilities are coming into focus. Here's an overview of what's on the radar screen.

The Highway 520 bridge replacement and highway expansion's most problematic section is on the Seattle side, between I-5 and Lake Washington. The final design and, therefore, the scope and impact of the project aren't yet determined, but a draft EIS by the Washington Department of Transportation outlines some of the areas where cultural resource issues are significant: the Montlake and Roanoke Historic Districts, the Arboretum, the Olmsted boulevard, even the mid-century modern 520 bridge itself. Of course, those are above ground.

From an archaeological perspective, the trickiest place is Foster Island, a cultural property important to numerous tribes. The Duwamish, Snoqualmie, Tulalip, Suquamish, Muckleshoot, and Yakama have all raised their hands and said they have a stake in it and their concerns about it have been described to me as "monumental." WSDOT is treating the site as significant enough to qualify for the National Register of Historic Places as a "Traditional Cultural Property," which is the bar to qualify it for special care and consideration.

Foster Island is reputed to be an old burial ground and a sacred site. Expanding a highway through an Indian graveyard is, problematic, to say the least. However, the exact nature of the burial ground is unknown.

There are reports that the bones were removed prior to the development of the Arboretum and the construction of the original 520 impacting the island, which was also previously logged. But there could still be remains buried there; tree burials often were moved underground. Out of consideration for tribal sensitivities, no one is going to dig or disturb the place if they don't have to. In other words, if the new bridge configuration can limit impacts on Foster Island, things will be easier for all concerned. But that's not assured, and all current options impact the island.

One of the interesting things about Foster Island is how little is known about it. Old maps record different sizes and shapes over the years, and it is thought to have been two islands at one time. Non-invasive ground testing shows that the current 520 runs more or less between two older islands that were connected when the Lake Washington Ship Canal opened in 1916 and dropped the level of the lake by nine feet. Some of the exposed mudflats were then covered with landfill. If the gap theory is right, burials and artifacts might not be found in the current footprint since it was once underwater. Once work begins, an agreement would be in place that would detail procedures in the event of an unanticipated discovery of human remains.

The history of the lake raises issues of its historic water levels and shifting shorelines. While Lake Washington was higher in 1916, the lowering of its water level then revealed prehistoric sites on its ancient shores. Some of these are referred to in an article by historian Coll Thrush, author of Native Seattle, in Pacific Historical Review in 2006.

In 1913, the digging of the Ship Canal's lockpits uncovered a deep shell midden and many ancient artifacts. When Union Bay was lowered, just across from Foster Island, the remains of ancient fishing weirs were uncovered at the site of a native town called Little Canoe Passage (roughly the site of present day University Village). Thrush writes that along the shorelines, ancient stone hearths were uncovered "laid millennia earlier when the lake had been an inlet of Puget Sound."

So, the water level was once as low or lower in the distant past, meaning there could be interesting stuff under the lake or on adjacent lands that could be uncovered during a major project. Add to this the fact that for centuries Montlake was a major native portage between Lake Union and Lake Washington (that's why they call it Portage Bay), and you have a zone of disturbance that could yield unexpected finds even outside of Foster Island. For example, according to WSDOT documents, there was reputedly a Duwamish longhouse at the mouth of the creek that ran through where the current Arboretum is to Lake Washington.