The origin, formation mechanism, and dynamics of suction vortices in a pump sump have been clarified by large eddy simulation (LES) applied to two different computational models. The first one is a pump-sump model with uniform flow entering a water channel of rectangular cross section and a vertical suction (outlet) pipe installed at its downstream end. LES with different wall boundary conditions have revealed that the origin of a submerged vortex is the mean shear of the approaching boundary layers that develop on the bottom and side walls of the sump. Detailed investigations have revealed that deviation of the mean flow triggers conversion of the vorticity axis to the vertical direction. The local acceleration of the vertical flow stretches the aforementioned vertical vortex, which results in the formation of a submerged vortex. The second one is a simplified computational model composed of a paraboloid of revolution and aims to accurately simulate the stretch of the viscous core of a submerged vortex that has appeared under the suction pipe of the pump-sump model. The differences between the models, especially predictions of the minimum pressure, imply that cavitation could have been initiated in the viscous core, if it had been taken into account, as is observed in the pump-sump experiment at the same condition. Parametric studies with different initial swirl numbers from 0.12 to 16.3 have clarified the behavior of the submerged vortex. It was found that a strong submerged vortex appears only at a relatively small range of the swirl numbers from 1.25 to 3.