Learn Russian: Город пустует, под ногами только пепел. - The city stands empty, under our feet is only ash.

It is the present tense of the verb пустовать (to be empty or to stand vacant), which is derived from the adjective пустой (empty). Russian frequently uses dedicated verbs to describe a state where English would use 'to be' plus an adjective.

Why is there no Russian word for 'our' in под ногами?

Russian typically omits possessive pronouns with body parts when the owner is obvious from the context. Additionally, под ногами is a common set phrase that translates directly to the English concept of 'underfoot'.

Why is ногами in the instrumental case here?

The preposition под requires the instrumental case when it describes a static location, answering the question 'where?'. If it were describing the direction of a new motion, answering 'where to?', it would require the accusative case instead.

Since this lesson focuses on verbless negation with ни + Genitive, how could we add 'Not a single person' to emphasize the emptiness of this sentence?

You would simply say Ни одного человека. To express the absolute absence of something, you do not even need the negative word нет. The particle ни paired with the genitive case creates a complete verbless negative clause on its own.