In physics, the term swampland is used in contrast to the term "landscape," to indicate physical theories or aspects of such theories which could be true if gravity were not an issue, but which are not compatible with string theory. Recent developments in string theory suggest that the string theory landscape of vacua is vast.
It is natural to ask if this landscape is as vast as allowed by consistent-looking effective field theories. Some authors (like Cumrun Vafa) suggest that this is not the case, and that the landscape is surrounded by an even more vast swampland of consistent-looking semiclassical effective field theories, which are actually inconsistent. Some proposed swampland criteria are:
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