I've wondered: Instead of drone delivery, why not use underground conveyer belts for delivery in communities with sufficient population density. Instead of delivery trucks burning carbon and adding traffic, and people carrying packages to and from them, deliveries could be loaded onto the conveyer system and routed to their destinations.

Imagine Manhattan with no more delivery trucks and all the deliveries, every day, for office buildings and apartment buildings being conveyed directly to their basements (and maybe even to an elevator, which dumps the packages on the appropriate floor).

The deliveries could be the mail and packages we are accustomed to now, but also anything else from one building to another intra-city, b2b, b2c, or c2c: Carryout meals (or a meal you made for your friend), groceries and drugs (especially good for people too ill to go out), a computer to be repaired, the ISP could send a replacement router, the papers you need from the office, a substitute for intra-city mail, boxes to UPS/Fedex/etc -- anything could be delivered within hours (though I might be overestimating how efficiently a network with so many nodes could route).

Perhaps it would be too costly, but building a conveyer belt under someplace as dense as Manhattan seems potentially cost-effective for all the benefits. And again, perhaps I'm underestimating the complexity of a network with that many nodes.

I realize this is a bit of blue sky thinking, and I know nothing of such technology. And I'm pretending that there are no political issues.