According to the Achilleid, written by Statius in the 1st century AD, and to non-surviving previous sources, when Achilles was born Thetis tried to make him immortal by dipping him in the river Styx; however, he was left vulnerable at the part of the body by which she held him: his left heel .
ɐkˈoːɹdɪŋ tə ðɪ ɐtʃˈɪleɪd, ɹˈɪʔn̩ baɪ stˈæɾɪəs ɪnðə fˈɜːst sˈɛntʃɚɹi ˌeɪdˈiː, ænd tə nˈɑːnsɚvˈaɪvɪŋ pɹˈiːviəs sˈoːɹsᵻz, wˌɛn ɐkˈɪliːz wʌz bˈɔːɹn θˈɛɾiz tɹˈaɪd tə mˌeɪk hˌɪm ɪmˈɔːɹɾəl baɪ dˈɪpɪŋ hˌɪm ɪnðə ɹˈɪvɚ stˈɪks; haʊˈɛvɚ, hiː wʌz lˈɛft vˈʌlnɚɹəbəl æt ðə pˈɑːɹt ʌvðə bˈɑːdi baɪ wˌɪtʃ ʃiː hˈɛld hˌɪm: hɪz lˈɛft hˈiːl .