Which is a simple sentence?

 - The American badger, one of the fastest digging animals on the planet, can dig quickly through both soil and concrete.
 - When the submarine surfaced, a crowd of people on the shore gathered and stared at the rare sight.
The second sentence is the simple sentence. It is a single independent clause.
The American badger, one of the fastest digging animals on the planet, can dig quickly through both soil and concrete.