We conducted a large scale experiment of underwater oil release of 6.3 L/s through a 25.4 mm (one inch) horizontal pipe. Detailed measurements of plume trajectory, velocity, oil droplet size distribution, and oil holdup were obtained. The obtained experimental data were used for the validation of the models JETLAG and VDROP-J. Key findings include: (1) formation of two plumes, one due to momentum and subsequently plume buoyancy, and another due mostly to the buoyancy of individual oil droplets that separate upward from the first plume; (2) modeling results indicated that the traditional miscible plume models matched the momentum and buoyancy plume, but were not able to simulate the upward motion plume induced by individual oil droplets; (3) high resolution images in the jet primary breakup region showed the formation of ligaments and drops in a process known as “primary breakup”. These threads re-entered the plume to re-break in a process known as “secondary breakup”; (4) the plume velocity was highly heterogeneous with regions of high velocity surrounded by stagnant regions for various durations. The results from this study revealed that the primary breakup is a key factor for quantifying the droplet size distribution which plays a crucial role in determining the ultimate fate and transport of the released oil in the marine environment. The observed spatial heterogeneity in the oil plume implies that the effectiveness of applied dispersants may vary greatly when applying directly in the discharged oil flow.