Abstract: On October 11, 2001, Employees #1 through #4 were conducting a 15,000-psi pressure test of a lubricator assembly on a rotary drilling rig. During the first attempted test, a two-in. nipple 1502 hammer union connection on the lubricator assembly failed. The subsequent sudden and violent release of pressure within the lubricator is suspected to have created a vacuum which caused a 3.125-in. OD casing collar locator tool (CCL) (which had been attached to the wireline within the assembly), to ascend with extreme force and lodge itself inside of a 3.0625-in. ID lubricator adapter spool, approximately 15 ft above the rig floor. Acting like a plug, the CCL trapped the remaining pressure below the point of its insertion. The failed connection was subsequently replaced and pressure testing resumed. After an apparent successful pressure test and pressure bleed off, the rig crew confirmed a zero pressure reading on the gauge attached to the side of the lubricator assembly and three remote gauges with independent readings were read by service company employees. The independent pressure readings were relayed to the rig crew before they returned to the rig floor. However, the points at which the readings were taken were located above the trapped CCL. Therefore, it appears the apparent successful pressure test, the subsequent pressure bleed off, and the equipment pressure readings were indicative of only that space above the CCL. When employees returned to the rig floor and began removing bolts between two spacer spool flanges, approximately 4 ft above the rig floor and below the trapped CCL, they were struck by pressure and fluid which had remained in the system below the point of the trapped CCL. Employees #1 though #3 were hospitalized with lacerations and other injuries. Employee #4 was treated for minor injuries and released.