The paper by Morrison and Vafa explores the compactification of F-theory on Calabi-Yau threefolds, focusing on the promotion of $N = 2$ dualities between type II/heterotic strings in 4 dimensions to $N = 1$ dualities between heterotic string and F-theory in 6 dimensions. They find that the six-dimensional heterotic/heterotic duality becomes a classical geometric symmetry of the Calabi-Yau in the F-theory setup. The authors also discuss the strong coupling transition and what lies beyond it, mapping it to a wall crossing in the Kähler cone of the Calabi-Yau. They derive the Calabi-Yau manifolds needed for F-theory compactifications, using results from algebraic geometry, and provide examples of these constructions. The paper includes detailed mathematical aspects of elliptic Calabi-Yau threefolds, including the conditions for existence of elliptic fibrations and the counting of complex structures. Finally, they explore the physical implications of the F-theory/heterotic duality, particularly the strong coupling phase transition and the heterotic/heterotic duality in 6 dimensions.The paper by Morrison and Vafa explores the compactification of F-theory on Calabi-Yau threefolds, focusing on the promotion of $N = 2$ dualities between type II/heterotic strings in 4 dimensions to $N = 1$ dualities between heterotic string and F-theory in 6 dimensions. They find that the six-dimensional heterotic/heterotic duality becomes a classical geometric symmetry of the Calabi-Yau in the F-theory setup. The authors also discuss the strong coupling transition and what lies beyond it, mapping it to a wall crossing in the Kähler cone of the Calabi-Yau. They derive the Calabi-Yau manifolds needed for F-theory compactifications, using results from algebraic geometry, and provide examples of these constructions. The paper includes detailed mathematical aspects of elliptic Calabi-Yau threefolds, including the conditions for existence of elliptic fibrations and the counting of complex structures. Finally, they explore the physical implications of the F-theory/heterotic duality, particularly the strong coupling phase transition and the heterotic/heterotic duality in 6 dimensions.