However, there are some words that could be used in specific contexts:
* Unrevealed: This suggests something that is not yet known, though it may be eventually revealed.
* Underground: This can refer to something hidden or kept from public knowledge.
* Under wraps: This phrase means something is being kept secret, often used in a more informal context.
Ultimately, the best substitute for "secret" starting with "U" depends on the specific context and the desired nuance.