Dame Clara Butt was a true Dramatic Contralto, with a wide range, and a luscious dark tone. Her recorded range is from E3 to Ab5, but various reports make it far wider. She regularly performed Schubert's Der Tod und das Mädchen, ending on the low D3. Sir Adrian Boult told Michael Scott that he heard her vocalizing "between four B flats."
In an article in The Mercury, "her great range...being from lower C in the bass clef to A above the treble clef." While there are other articles which describe the full extent of her range as F2 (Sarastro's lowest note) to C6!
In Liza Lehmann's The Birth of Flowers, Clara holds a sustained C3, truly the lowest note of hers on any recording found so far.
The C3 starts at 3:16. Enjoy!