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The Core Stage of the new NASA Space Launch System (SLS) is a 212-foot tall rocket assembly—consisting primarily of two fuel tanks, an engine section, and four RS-25 rocket engines—capable of sending crew and large payloads to the moon and beyond with 1.6 million pounds of thrust. Currently, the Core Stage is in the B2 Test Stand at Stennis Space Center, undergoing a series of structural and functional tests, designated the Green Run test series. The ultimate goal of Green Run is to verify analytical models and confirm proper subsystem operation of the Core Stage. In January 2020, Green Run testing began with an experimental modal analysis test performed by the Marshal Space Flight Center modal test team. The desired free boundary condition of the Core Stage was achieved by suspending the massive spacecraft from the B2 Test Stand crane. Modal excitation was provided by a pair of 250-lb electro-dynamic shakers for multi-shaker random vibration testing, as well as a 12-pound impact hammer for impact testing. Modal response was measured with 550 accelerometers channels distributed on both the Core Stage and the B2 Test Stand crane. Following one very long night of testing, frequency response functions were calculated from the measured time histories in the target mode frequency range of 2.5 Hz to 15 Hz, and mode shapes, frequencies, and damping values were successfully extracted. The case-study presented in this paper will discuss the SLS Core Stage, the modal test setup and procedure, as well as a brief overview of the test results. Challenges associated with testing such a large, suspended structure in an outdoor environment will be discussed as well.