In 2000, Jill Heinerth was already a renowned diver, known for her exploits mapping vast underground cave networks in Florida. Filming for a National Geographic documentary brought a new and unprecedented challenge; a vast iceberg known as B-15 had broken away from an ice shelf in Antarctica, providing a unique chance to explore its networks of underground caves. Braving sub zero temperatures and the treachery of constantly shifting ice, Jill became the first person to ever enter one of these caves - a historic milestone in diving. She tells her story to Emily Finch.

PHOTO: The B-15 iceberg (Wolfgang Kaehler/LightRocket via Getty Images)