After hearing the report of their Trojan companions, Achates impresses upon (compellat, 581) Aeneas that things stand as Venus had said, presumably because he believes it is time for them to be revealed (we know from 580-1 that they both were eager to become visible again). Here is what he says:
“Nate dea, quae nunc animo sententia surgit?
Omnia tuta vides, classem sociosque receptos.
Unus abest medio in fluctu quem vidimus ipsi
summersum; dictis respondent cetera matris.” (1.582-5)
The words medio in fluctu are placed oddly: they belong in the relative clause introduced by quem, but they have been pulled out of it. Why? For one thing, it serves to draw added attention to the one man who has been lost, Orontes, as does the enjambed summersum in the next line. Both are, in a sense, displaced, as was poor Orontes. Also, the word-order allows Vergil to put the phrase for the “middle” (of the wave) in the middle of the line. The elision of medio with in is also effective to mimic aurally the drowning of the man.
A couple of other notes: there is a nice contrast between line-initial omnia and unus in consecutive lines: “all are safe, one excepted.” The contrastive parallelism is accomplished also through the double use of videre: “you see all things safe [now], except for the one man we saw drowned [then].
Finally, there is a nice contrast between the quotation and its frame. The description of the death of Orontes has to do with concealing and disappearance (summersum). The quotation comes as a transition between the concealment of Aeneas and Acestes, who were already desiring to be revealed (jamdudum erumpere nubem/ardebant, 580-1), and their bursting into appearance from behind the cloud (repente/scindit se nubes et in aethera purgat apertum, 586-7). Within the quotation itself, the opinion of Aeneas “rises” (surgit) in contrast to Orontes, who sinks.