ɪn nˈaɪntiːnhˈʌndɹɪd sˈɛvənti fˈaɪv ðɪ ˈɛs twˈɛnti wˈʌn kɹˈæʃt dˈʊɹɹɪŋ tˈɛstɪŋ, lˈeɪɾɚ pɹˈuːvd təbi dˈuː tʊ ɐ dᵻfˈɪʃənsi ɪnðɪ ˈælɔɪ jˈuːzd fɚðə hˈʌl.
In 1975 the S-21 crashed during testing, later proved to be due to a deficiency in the alloy used for the hull.