Learn Spanish (Spain): Ese es el desierto más seco de España.

To say "the most [adjective]" (a relative superlative), Spanish uses the formula: definite article + noun + más + adjective. So here we have el (the) + desierto (desert) + más (most) + seco (dry). Literally, it reads as "the desert most dry".

Why does the sentence use de España instead of en España for "in Spain"?

In English, we say the driest desert "in" a place. But when using superlatives in Spanish, you always use the preposition de to define the group or location. You can think of it as "the driest desert of Spain".

Why does the sentence start with Ese instead of Eso for "That"?

Because "that" refers directly to a specific masculine noun: desierto (desert). We therefore use the masculine pronoun ese. The neuter pronoun eso is only used for abstract ideas, general statements, or when we don't know what the object is yet.

What would this sentence look like if we were talking about a feminine noun, like la zona (the zone)?

Everything that refers to the noun would need to change to feminine. It would be: Esa es la zona más seca de España. Notice how ese, el, and seco all change to their feminine forms to match zona.