Water from outside
Grading that slopes toward the foundation, blocked downspouts and window wells without drainage all direct rainwater to the walls of the lower level, where it can enter through cracks and joints.
Water from the drainage system
Sump pump failures, power interruptions and drain or sewer backups can introduce large volumes quickly. Where waste water is involved, the cleanup is treated as a contamination issue.
Water from inside
Mechanical rooms hold water heaters, laundry connections and supply lines. A failure at any of these can flood a finished basement without any weather involved.
This guide is general information. Conditions vary between properties, so decisions about your building should follow an assessment of the affected areas.