I'm interested in any historical examples people have of altruistic actions taken primarily to benefit future generations, which don't have strong positive effects for current generations. (So e.g. most climate change mitigation efforts wouldn't count, if we expect climate change to have a big negative impact on people currently alive - but maybe some do?)
Maybe some construction megaprojects might count, I'm thinking the Notre-Dame Cathedral which took about 100 years to complete.
This might not really count because the choir was completed after about 20 years. I'm also not sure if it was meant to take so long.