Alternative 2 was a plan to construct vast subterranean habitats for the elite to escape into when the going gets tough*
*But why construct something which is already there? For aeons, such vast subterranean realms have existed. There is literally a million miles of tunnel systems beneath our feet. Along with unpublicized transportation which uses these networks. Transportation technology far in advance of the fossil fuel primitive transportation which exists on the planet.
There are unseen civilisations and people's below the earth. Perhaps this is the reason why Alternative 2 has given way to alternative 3?