From mathematician Peter Woit at Not Even Wrong:

Way back in 2005, soon after the emergence of the “String Landscape” and the ensuing debate over whether this made string theory untestable pseudo-science, Cumrun Vafa in response started writing about the “Swampland”. In contrast to the “Landscape” of effective field theories that are low energy limits of string theory, the “Swampland” is the space of effective field theories that are not low energy limits of string theory. One motivation here is to be able to claim that string theory is predictive, since if you can show a theory is in the Swampland, then string theory predicts that doesn’t describe our world.

I wrote a couple blog postings about this back then, see here and here. The situation was rather comical, with Jacques Distler unintentionally making clear one problem with the whole idea. More.

String theory is not something to which evidence makes any difference. It is true because proponents need it to be. They speak for fashionable science, an enterprise they own, along with Darwinism and the multiverse.

*See also:* Post-modern physics: String theory gets over the need for evidence

To me, this is the nub of the problem with quantum gravity, something string theory seems to give us:

Since 1905, physics has, IMHO, misunderstood the “vacuum,” which is really another word for the “aether.” Armed in your considerations with

Heisenberg’s ‘Uncertainty Principle,’ at smaller and smaller scales–half of the (delta x)*(delta p) of the ‘uncertainty principle,’ you find that momentum (p) becomes very large and, in this sense, violent, or chaotic.

Comfortable with the notion of the “vacuum,” which, BTW, gives you the single greatest error in calculations anyone has seen (the energy of the ‘vacuum’), no acceptable theory of QG will be found. I suspect that it will be the solid-state physicists who will, via experimentation, discover the proper understanding of the “aether/vacuum,” just as they led the way in pointing to the theoretical understanding of the weak force.