# Variations in which feature(s) of string theory give rise to so many possible Universes?

The string theory landscape comprises $10^{500}$ (give or take some) different Universes. What feature(s) of string theory you have to vary to "go" from one Universe to the next?

$10^{500}$ is an approximate number of possible $6D$ Calabi-Yau compactifications of String Theory, which is purely geometrical fact (look at this book on Mirror Symmetry for a very detailed and thorough discussion of the subject). For lower dimensional compactifications the situation is much simpler -- just $S^1 \times S^1$ for $2D$ case, and $S^1 \times S^1 \times S^1 \times S^1$ or so-called K3 manifold for $4D$ case.
Calabi-Yau compactifications are relevant, if one restricts the VeV's of background fields to be zero. In principle, it doesn't have to be the case, and there are many very interesting and important solutions of String Theory with non-zero background fields. For example, Type IIB String Theory on $AdS_5 \times S^5$ background with non-zero flux of 5-strength of 4-form field through $S^5$, which appeared to be dual to ${\cal N}=4$ Super Yang-Mills theory in $4D$. There are also many possible flux compactifications in the form $\mathbb{R}^4 \times M_6$, where $M_6$ is a compact manifold with non-zero fluxes of certain String Theory fields through it (see, for example, this review).