Learn French: Selon notre source, l'accès serait sous le musée. - According to our source, the access is reportedly under the museum.

This is the "journalistic conditional." By using the conditional serait (would be) instead of the present tense est (is), the speaker shows that this information is unconfirmed. In English, this is often translated using words like "reportedly" or "supposedly."

Does accès just mean "access" as an abstract concept?

It can, but in physical contexts like this, un accès often translates better as "an entrance" or "a way in." Here, they are talking about a physical entry point located under the museum.

How do you use the word selon?

Selon means "according to." You can use it directly with a noun, like selon notre source (according to our source) or selon les journaux (according to the newspapers). You can also use it with a stressed pronoun to say things like selon moi (according to me) or selon eux (according to them).

Why is it sous le musée and not sous du musée?

The preposition sous (under) is followed directly by the noun and its article. It does not take de (of), unlike some compound prepositions you might remember such as à côté de (next to) or près de (near).