Answer:
Underneath the mattress, Flash hid a secret envelope.
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.
<em>Underneath</em> is a preposition of space, just like some other prepositions, such as: <em>in, on, under, below, above, behind, in front of, between,</em> etc.
Some other prepositions are: <em>from, towards, to, by, in, until, till, at.</em>
