No doubt about it, the National Archives are intimidating. And it's not the kind of place to wander in and ask, "Whaddaya got?" But when you're ready with specific questions, you can start with the Great Lakes Region in Chicago. Call first and talk to an archivist.
What can you find there? Absolutely anything, and not necessarily where you expect. The National Archives' official motto is "What Is Past Is Prologue," but a case could be made for changing it to "Who Woulda Thunk It?"
An article in the Great Lakes Region's February 2009 monthly newsletter (not yet on line) describes the paper trail created when the federal government sold off its holdings on Grosse Isle in the Detroit River after World War II and hired a title company to do a search. That file included a photocopy of a 6 July 1776 treaty or deed to Alexander and William Macomb and signed by several Potawatomi chiefs:
Chief Magina's seal is an upside down deer and Chief Nanakota's seal is a fish with a very distinctive crosshatch pattern. The final pictograph, a tent, is that of Wabateathaque; his is the largest and closest to the signatures of the English.
Not just amazing, but conceivably of genealogical use if you need to confirm an 18th-century Native American identity by matching signatures. The citation is Grosse Ile Naval Air Station - Real Property Disposal Case Files. Records of the Chicago Regional Office. Accession RG 291-75A-0238-Box 25 Folder 15. Records of the Federal Property Resources Service. Record Group 291. National Archives-Great Lakes Region (Chicago).