With any POD between now and the late 1800s (but preferably sometime in the 1900s-1930s, hence its place in After 1900), make it relatively common for governments and any other suitably large groups to fund the construction of vast projects like an early Channel Tunnel (which was actually planned at some point in the 1800s IIRC), damming the Red Sea, or following through with any one of Frank Lloyd Wright's or Buckminster Fuller's ideas.
Preferably I'd like for history to progress to the point that in the '80s-'00s the world would have gotten used enough to this sort of thing that an attempt at Atlantropa would be seen as possible.*
*Let's assume that nobody involved would have any issues with it besides the feasibility of the thing and there's nobody talking about how Atlantropa would destroy so-and-so's economy.