Simplify the following text for a Grade 8 native English-speaking person.
Original: The Mariner , who can breathe underwater , puts her in a diving bell , and swims down to the ruins of a city that was once Denver and grabs a handful of dirt .
Simplified: The Mariner puts her in a diving bell made out of plastic and swims down to a sunken city ( actually the city of Denver ) to show her .