Reagan National Airport was actually built on swamp land filled in with dirt. Since the swamp land was part of the Potomac River (DC Territory). The District of Columbia has been trying to claim Reagan National Airport as theirs so they can gain tax benefits.
http://en.wikipedia.org/wiki/Ronald_Reagan_Washington_National_Airport
Dulles- By operation of the transfer act, all property owned by the FAA at the airports was transferred to the MWAA, with the Federal government retaining title to the lease. The original 50 year lease has been extended to 80 years, expiring in June, 2067. Dulles straddles the border of Fairfax County and Loudoun County, Virginia. Depends on which terminal you are in, you can be in Fairfax County or Loudon County.(This is how Taxes/permits work, not sure if this applies to laws)