There are two lighthouses in the old port area in La Rochelle, and when you are travelling into that port area, you need to line them up so that one light is above the other; then you are on the correct channel to come into La Rochelle. The channel is prone to silting up.

I find the ideas that go into this sort of planning/engineering fascinating. I wonder if we would think of these things if we were trying to resolve this problem today.