Caves' as in most plural possesives.
E.g. The Mendip Hills' caves' streams were traced in the Mendip Karst Hydrology Project.
Though that example (referring to a study carried out in the 1960s) would be more euphonious ifcaves' was replaced by cave,turning the word from noun to adjective and removing an awkward conjoined-s.)
Singular possessive: secretary's Plural: secretaries Plural possessive: secretaries'
The plural is relatives. The plural possessive is relatives'.
The plural is lads. The plural possessive is lads'.
The plural form is echoes. The plural possessive is echoes'.
The plural form is suffixes. The plural possessive is suffixes'.
The plural form is Mexicans. The plural possessive is Mexicans'.
The plural form is stepsisters. The plural possessive is stepsisters'.
The plural form is inches. The plural possessive is inches'.
The plural possessive of "Father" is, "Fathers"
The plural possessive of bandit is bandits'.
Studies' is the plural possessive of study.
Coaster's is singular possessive. The plural is coasters, the plural possessive is coasters'.