Landau Singularities of the 7-Point Ziggurat I

We compute the leading (first-type Landau) singularities of a certain four-loop 7-point graph that is related to the 7-point ``ziggurat'' graph by the graphical moves familiar from equivalent circuit theory. We find perfect agreement with a subset of the ``heptagon symbol alphabet'' that has appeared in the context of planar $\mathcal{N}=4$ super-Yang-Mills theory. The remaining heptagon symbol letters are found in its subleading Landau singularities, which we address in a companion paper.