The response of the pressure-suppression containment system of Mark I boiling water reactors to a large break, design basis loss-of-coolant accident was examined on a scale experimental facility. A logical interrelationship between measured forces, measured pressures, and the hydrodynamic phenomena (observed with high-speed cameras) is established. Peak downward forces on the wetwell occur at about the time of vent clearing. Peak upward forces occur shortly before bubble breakthrough. Quantitative values of forces and pressures from our scale experiment, at times up to and including the peak download, can be applied to full-scale plants using established scaling laws. Only qualitative relationships to full-scale plants are determined at later times, because substantial quantities of steam (not simulated in our scale experiment) would have entered the wetwell.