[en] Material Disposal Area B (MDA B) is the oldest radioactive waste disposal facility at Los Alamos National Laboratory. Operated from 1944-48, MDA B was the disposal facility for the Manhattan Project. Recognized as one of the most challenging environmental remediation projects at Los Alamos, the excavation of MDA B received $110 million from the American Recovery and Reinvestment Act of 2009 to accelerate this complex remediation work. Several factors combined to create significant challenges to remediating the landfill known in the 1940's as the 'contaminated dump'. The secrecy surrounding the Manhattan Project meant that no records were kept of radiological materials and chemicals disposed or of the landfill design. An extensive review of historical documents and interviews with early laboratory personnel resulted in a list of hundreds of hazardous chemicals that could have been buried in MDA B. Also, historical reports of MDA B spontaneously combusting on three occasions -with 50-foot flames and pink smoke spewing across the mesa during the last incident in 1948-indicated that hazardous materials were likely present in MDA B. To complicate matters further, though MDA B was located on an isolated mesa in the 1940's, the landfill has since been surrounded by a Los Alamos commercial district. The local newspaper, hardware store and a number of other businesses are located directly across the street from MDA B. This close proximity to the public and the potential for hazardous materials in MDA B necessitated conducting remediation work within protective enclosures. Potential chemical hazards and radiological inventory were better defined using a minimally intrusive sampling method called direct push technology (DPT) prior to excavation. Even with extensive sampling and planning the project team encountered many surprises and challenges during the project. The one area where planning did not fail to meet reality was safety. There were no serious worker injuries and the minor injuries recorded were those common to construction type activities. Extensive monitoring along the site boundary demonstrated that no hazardous chemicals were released and radiological dose to the public was within administrative limits. More than three years of effort by the LANL project team went into the planning for remediation of Material Disposal Area B. Hundreds of historical documents were reviewed; retired personnel were extensively interviewed and noninvasive techniques were used to characterize the site. The information collected was incorporated into the safety requirements, cost estimate, schedule and primary execution plan for the project. Ultimately the waste volume managed by the project approached 40000 m³, more than double the original project estimate. This increase had a major impact on both project cost and schedule. Nuclear safety requirements for the project were based on an estimated MDA B radionuclide inventory of 12 PE-Ci. When excavation was complete over 123 PE-Ci had been removed from the trenches. The radionuclide inventory at MDA B was an order of magnitude higher than estimated. Work at MDA B could not have proceeded without the safety basis exemption from DOE-HQ. The one area where planning did not fail to meet reality was safety. There were no serious worker injuries and the minor injuries recorded were those common to construction type activities. Extensive monitoring along the site boundary demonstrated that no hazardous chemicals were released and radiological dose to the public was within administrative limits. (authors)