One person was inside the crawl space under the house after dark to observe, one person was outside shining a bright light towards the crawl space, at ground level. The person on the inside saw the brightest light at one particular spot, and some light at some other spots along a horizontal line at ground level. This shows that the foundation is porous along that edge of the house -- which we already suspected since this is the area where a lot of water comes into the cellar after a heavy rain.
What are the possible approaches to remediating the porousness? Note that that edge of the house doesn't feature a full foundation. That portion of the house was originally a porch, and later the porch was remodeled and added on to. The edge in question is several feet out from the original porch boundary.