I. INTRODUCTION II. BACKGROUND AND SUMMARY OF MARSH A. Duty to Prepare an EIS: A Requirement B. Duty to Prepare a Supplemental EIS: The Marsh Legacy C. A Closer at Marsh: A Dam and Two Documents III. APPLICATION OF MARSH IN THE U.S. COURTS OF APPEALS--WHAT KINDS OF NEW INFORMATION DO COURTS FIND (AND NOT FIND) SIGNIFICANT? A. Hold the Phone! Supplementation (or at least more review) is Required B. Nothing to Supplement Here 1. Information Previously Considered 2. Speculative Environmental Impacts 3. Information does not Undermine Assumptions in EIS 4. Common Sense 5. Impacts will Ultimately be Studied Elsewhere 6. Summary IV. THE MARSH TWO-STEP? A. The Basis for the Two-Step Review in Marsh B. Whether the Two-Step Marsh Test Undermines NEPA's Rule of Reason C. Application of the Hard Look Principle Going Forward D. Steps to Minimize Legal Risk 1. Marsh: the Paradigmatic Hard Look 2. Hughes River Watershed--an Incomplete Review 3. The Importance of Public Involvement 4. Pragmatic Approaches to New Information 5. Choreographing the Marsh Two-Step (and One-Step) V. CONCLUSION NEPA ensures that the agency will not act on incomplete information, only to regret its decision after it is too late to correct. *** I. INTRODUCTION We live in a world of information overload. Smartphones, Facebook, Twitter, and email alert us to breaking news instantaneously. Almost before we process new information, another sensational story takes center stage: the latest political scandal, mass shooting, plane crash, superstorm, kidnapping, or act of war or terror. How do we react to this ever-evolving information? Some people cancel travel plans, remove children from public school, or buy security systems or weapons. others move on with their day and barely bat an eye as they process all of this new and seemingly information. For federal agencies preparing environmental impact statements (EIS) under the National Environmental Policy Act (NEPA), the answer appears clear. Twenty-five years ago in Marsh v. Oregon Natural Resources Defense Council, (1) the Supreme Court held that an agency must supplement its EIS before taking federal action if it discovers new and information. (2) But what is significant? One might assume that it would be shocking events, like terrorism, nuclear accidents, and environmental disasters. However, as discussed below, courts rarely consider this type of information significant. Instead, it is the agency's response to new information that frequently proves decisive. Given the prevalence and importance of this issue, Marsh has often been cited over the past twenty-five years. (3) As major federal actions regularly take years to complete--occasionally years more than anticipated--agencies routinely confront the question of whether the analysis in an aged EIS is still legally sufficient, or whether the issuing agency must update or supplement it with new information. (4) Unfortunately, a review of scholarly articles and case law reveals that this question has not been treated with the care it deserves. Few academic articles seriously discuss the problem of supplementation. (5) Worse, most reviewing courts routinely provide a cursory recitation of Marsh boilerplate and then leap to a conclusion of whether an agency acted correctly. (6) In honor of Marshs twenty-fifth anniversary, this article takes a fresh look at this Supreme Court decision and the Federal courts' subsequent reactions to it. First, we examine what actually constitutes new and information under Marsh. While courts do not crisply define significant, (7) a review of the courts' approaches to the problem may help determine what information is actually significant in practice. Second, we examine what procedural steps an agency must take to determine whether new information actually meets the significant standard. …