À la sortie du tunnel, on aperçoit déjà l’horizon au-dessus de la mer.

It means something like at the exit of the tunnel, when coming out of the tunnel, or upon leaving the tunnel.

In French, à la sortie de... is a very common pattern for talking about the moment or place where something comes out of somewhere:

• à la sortie de l’école = when leaving school / at the school exit
• à la sortie du métro = when coming out of the metro

So here it sets the scene: once you reach the tunnel exit, you can already see what follows.

Because de + le contracts to du in French.

So:

• de le tunnel → du tunnel
• de les → des

This is a standard contraction:

• la sortie du tunnel = the exit of the tunnel
• le toit du bâtiment = the roof of the building

You cannot normally keep de le in standard French.

On is extremely common in French. Here it has a general meaning, like:

• you
• one
• people
• sometimes even we

In this sentence, on aperçoit déjà l’horizon... means something like:

• you can already see the horizon...
• one can already make out the horizon...
• the horizon is already visible...

French often uses on where English would prefer a more general subject or even a passive-style translation.

Because on always takes a third-person singular verb form.

So:

• on aperçoit
• on voit
• on arrive

Even when on means we in everyday French, the verb still stays singular:

• On arrive bientôt = We’re arriving soon

That is why it is aperçoit, not aperçoivent.

Apercevoir often means to catch sight of, to make out, or to glimpse. It can suggest that something is becoming visible, perhaps at a distance.

So:

• voir = to see
• apercevoir = to catch sight of / make out

In this sentence, on aperçoit déjà l’horizon feels a little more descriptive than just on voit déjà l’horizon. It suggests the horizon is now coming into view as you leave the tunnel.

Déjà means already.

Here it adds the idea that as soon as you come out of the tunnel, the horizon is visible right away, perhaps sooner than expected.

So the sentence is not just saying that the horizon is visible. It is saying that it is visible already, at that point.

Because le becomes l’ before a vowel sound.

• le horizon → l’horizon

This is called elision.

Also, horizon begins with an h muet in French, which behaves like a vowel for this purpose. So you must say:

• l’horizon
• not le horizon

The same thing happens in:

• l’homme
• l’hôtel
• l’histoire

Au-dessus de means above or over.
Sur usually means on or on top of.

So:

• au-dessus de la mer = above the sea
• sur la mer = on the sea / on the surface of the sea

For the horizon, au-dessus de la mer makes sense because visually the horizon appears above the sea line from the viewer’s perspective.

Using sur la mer would sound different and would not express the same spatial idea.

Because mer needs its article here: la mer.

French usually uses an article where English may not:

• the sea = la mer
• the horizon = l’horizon

So the structure is:

• au-dessus de la mer = above the sea

Also, au-dessus de is a fixed expression followed by de + noun phrase.

Aperçoit is in the present tense.

French often uses the present tense to:

• describe what happens in a scene
• give a vivid, immediate impression
• state something generally true or visually observable

So here the present tense makes the sentence feel direct and scenic, as if the reader is experiencing the moment right now.

Yes, and it would be close in meaning, but not exactly the same in feel.

• À la sortie du tunnel = at the exit of the tunnel / upon coming out of the tunnel
• En sortant du tunnel = while coming out of the tunnel / as you come out of the tunnel

The original version is slightly more noun-based and descriptive, focusing on the exit point itself.
The en sortant version feels a bit more verbal and action-based.

Both are natural, but the original sounds very idiomatic and well suited to descriptive writing.

The structure is:

À la sortie du tunnel,
= introductory prepositional phrase setting the scene

on aperçoit
= subject + verb

déjà l’horizon
= adverb + direct object

au-dessus de la mer
= prepositional phrase describing where the horizon appears

So the sentence is built like this:

[When/where?] + [Who/what?] + [Does what?] + [What?] + [Where?]

That kind of structure is very common in French descriptive sentences.