I live in a local minimum for my neighborhood, and in fact my house is built on the banks of a stormwater drainage ditch. Most of the rainwater from the road is collected by stormwater drains and fed directly into the ditch via a pipe, but the water that falls directly on our own and our neighbors' yards still has to make its way around (or through) our house to get to the ditch.
The house was built in 1920 and originally had a limestone wall foundation, parts of which have been replaced over the years - mostly with CMUs on a poured footer.
At the lowest end of the yard is the entrance to our crawl space. It appears that the foundation wall that runs along the slope (perpendicular to the ditch) was replaced with CMUs at some point, but it has another interesting feature - a large L-shaped concrete pour:
My guess is that they temporarily supported the house on the smaller pour+embedded deck block you see immediately to the right while they poured this footer. In fact when we bought the house there were still some posts standing on those blocks, though they were loose and no longer acting as support.
During the wetter months (winter/spring), this area is quite muddy (though no standing pools of water usually). My theory is that the footer is too shallow, so water is migrating under it and wetting the basement. The fact that there is a tree root growing through that space seems to give further credit to this theory.
As you can see, inside the crawl space the terrain follows the natural slope of the yard. But on the outside, a retaining wall made of railroad ties holds a large amount of soil up against the outside of the foundation, rather than letting the yard slope down naturally towards the ditch at the bottom:
My thought is that the retained soil on the outside acts as a sponge, and then slowly releases this moisture into the crawl space. Would making the footer deeper help keep the water out? If so, what is the best way to do this? My current plan is to reinstall the temporary supports, excavate the soil from around and under most of the footer, add a rebar grid (or cages) and build a plywood form, and then pour concrete. The hope is that the footer will support itself during this process with the "leg" at the lower end, and the unexcavated soil at the upper end.
Or should I simply remove the railroad ties and excavate the retained soil on the outside of the house, restoring the natural grade of the yard? I have considered replacing it with a deck on posts in this case.