Learn French: Ce tunnel serait un passage secret. - This tunnel is reportedly a secret passage.

serait is the conditional form of être (to be). It is used here to express unconfirmed information, a rumor, or an allegation. This is known as the journalistic conditional, which is why it translates to "is reportedly" or "is allegedly" rather than a factual "is".

If I didn't want to use the conditional, how else could I express this unconfirmed fact?

You could use the present tense along with phrases that show uncertainty, such as On dit que ce tunnel est... (They say that this tunnel is...) or Il paraît que ce tunnel est... (It seems that this tunnel is...). Using serait is just a more concise way to do this.

Why is secret placed after the noun passage?

Most French adjectives go after the noun they describe. The main exceptions are short, common adjectives relating to Beauty, Age, Number, Goodness, and Size (often remembered by the acronym BANGS). Since secret does not fit into those categories, it follows the standard rule and goes after passage.

Why do we use ce for "this" instead of cet?

Tunnel is a masculine singular noun that starts with a consonant, so it takes ce. The form cet is only used for masculine singular nouns that start with a vowel or a mute H (like cet arbre or cet homme) to make pronunciation smoother.