What is the subordinate clause in this sentence?

As I was walking, I stumbled upon a beautiful seashell.

The subordinate clause in this sentence is "As I was walking."

To identify the subordinate clause, you can follow these steps:
1. Identify the main clause in the sentence. In this case, the main clause is "I stumbled upon a beautiful seashell."
2. Look for a phrase or clause that does not make sense on its own and depends on the main clause to provide complete meaning. In this case, "As I was walking" is a dependent clause that does not make sense on its own.
3. Confirm that the dependent clause starts with a subordinating conjunction. In this case, the subordinating conjunction is "As."

So, "As I was walking" is the subordinate clause in this sentence.