Wikipedia Megaproject lists 200 currently viable human efforts costing over 1 Billion US dollars. Of the UNESCO – World Heritage Sites many of which were even greater human efforts when they were built only those still functioning economically appear to be listed. Ancient projects such as pyramids of Egypt, Meso America, Bolivia, Mesopotamia and elsewhere are not listed but they are still functioning as intended. Upgraded old water canals such as China’s Grand Canal, Suez Canal and Panama Canal are listed. The Great Wall of China, and Hadrian’s Wall are listed even though they no longer serve their design function and are presently only tourist attractions.
When one looks forward 5000 years, the same time as back to the Egyptian Pyramids, to a high-tech society, it will probably be wonderful in some ways but for some things like the aviation industry without doubt it will be a real downer. The enormous supplies of virtually free hydrocarbons such as gas, oil, and coal which modern civilization thrives upon will be gone. They will be gone long before humans have finally gone extinct and those future people will be forced to rely on renewable resources derived more directly from the sun such as plant life, solar collectors and wind. Hydrocarbons will simply be too scarce to burn up jetting about for vacation entertainment. If that turns out to be true what might be the megaprojects currently under construction which will still be considered useful in 5000 years.
The Wikipedia’s Megaproject list:
Aerospace projects and the Airport projects are totally non-viable in 5000 years because they consume too much hydrocarbons.
Building projects, such as the various super-tower buildings will be long gone and seen as arrogant monstrosities of people with too much money.
Canal projects may well be of real value to distant people because bulk products can be moved great distances efficiently using them.
Dam projects may still be viable even though the reservoirs were long silted to their tops because some of the power generating capacity will still be available.
Defense projects will have destroyed the world as we know it and will be condemned and controlled or civilization won’t exist.
Sports projects like the Olympic Games may continue but not in the current stadiums but as traditional events in new stadiums built to permanent standards.
Internet Technology will be of much higher quality than we presently experience and available to everyone but invisibly monitored.
Oil, Gas and Coal projects will be still be coming out of the earth in smallish quantities, not for burning but for essential chemicals and food fertilizers.
Port projects will have all been built long ago and linked to the inland cities not so much with paved highways as at present but with electric railroads and water canals.
Rail and rapid transit projects will be in great use with an interconnected world-wide electrical system transporting people and goods intercontinentally probably even across the Bering Straights. Super trains are about one quarter as fast as current airliners and the routes are a little longer but still at 300 km per hour it’s only 16 hours coast to coast in the US and you are in the city not an airport. Quite a lot of the critical infrastructure, mountain pass cuts, tunnels and bridges when maintained will still be viable. Two thousand-year old Roman stone bridges are still being used. These types of structures should be engineered to last for thousands of years not tens of years.
Road traffic projects like many fabulous bridges now being built for automobiles will not last very long nor will the gasoline driven automobile but electric ones may have a permanent place. Much of their infra-structure may be converted to electric railways because they use less energy.
Science projects will evolve to things undreamed of but the ones now being funded will have answered all that can be asked of them. The genome project will bring us wonderful monsters which will serve humans in many unknown ways. However, our current weapons research will be brought to an absolute halt and the methods for making weapons of Extermination (WOEs) will be suppressed and forgotten but watched. The genetic knowledge has the power to create wonderful things but it also will have the power to target specific species and subspecies for extermination. That knowledge must be hidden and those who seek that knowledge must be destroyed — or our species goes extinct.
Urban projects are always going forward but when one thinks of a five thousand-year permanency of those projects a more solid and static world evolves. When the intent is to make the habitations as livable and comfortable as possible for humans a style people will enjoy improves their lives.
Water infrastructure projects are among those which should be built to thousand-year standards because civilization is as dependent on water as it is on food. Sewage treatment is the flip side of water availability and should also be designed for permanency of operation and with no outside energy input or human maintenance for its perfect operation.
Climate and environmental change projects, are a new effort on humans part to control in a positive way the ecology of very large portions of the Earth. Ways of taking CO2, and other noxious gasses out of the atmosphere and sinking them into permanent reservoirs may return the whole planet to a better place. Ways of cleaning the oceans themselves of toxins and maintaining what is found to be an ideal temperature.
Unmentioned in the Wikipedia Megaproject site was the biggest megaproject of all — bringing the human population to a permanently stable number.