That sentence is a famous test case because it’s grammatically odd in a subtle way.

The problem is:

- **“scientist discovered”** should be **“scientists discovered”** or **“a scientist discovered”**
- **“a herd of unicorns”** is fine
- the rest is mostly okay, though **“previously unexplored valley”** could be made smoother

A corrected version could be:

> In a shocking finding, scientists discovered a herd of unicorns living in a remote, previously unexplored valley in the Andes Mountains.

Or:

> In a shocking finding, a scientist discovered a herd of unicorns living in a remote, previously unexplored valley in the Andes Mountains.

If you want, I can also explain why this sentence became famous in NLP/grammar discussions.
