Answer:
Underneath the mattress, Flash hid a secret envelope.
Step-by-step explanation:
Prepositions are a class of words that usually stand in front of nouns, pronouns, or gerund verbs, used to express spatial or temporal relations or various semantic roles.
Underneath is a preposition of space, just like some other prepositions, such as: in, on, under, below, above, behind, in front of, between, etc.
Some other prepositions are: from, towards, to, by, in, until, till, at.