Also known as celery root, knob celery and turnip-rooted celery, celeriac developed from the same wild species as did stalk celery.

NPR: The Vegetable World's Ugly Duckling: Celeriac

Celeriac is a celery variety refined over time to produce an increasingly large, solid, globular root just below the soil surface.

NPR: The Vegetable World's Ugly Duckling: Celeriac