moles

Moles, molis is third declension feminine. The nominative singular moles and the nominative and accusative plural moles are identical, which is the usual third declension nuisance; the accusative singular molem is the form that tells you where you are.

It covers three related things: sheer bulk, a massive built structure such as a mole or breakwater or dam, and the effort that shifting such a thing costs. That last sense gives Virgil's tantae molis erat Romanam condere gentem, "so great a task it was to found the Roman race", where moles is a genitive of quality and means trouble, not rock. Do not confuse it with mola, "millstone, mill", nor with molestus, "troublesome", which is a genuine cousin and the source of English molest.