We present measurements of the atmospheric depth of the shower maximum <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi>X</a:mi><a:mi>max</a:mi></a:msub></a:math>, inferred for the first time on an event-by-event level using the surface detector of the Pierre Auger Observatory. Using deep learning, we were able to extend measurements of the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msub><c:mi>X</c:mi><c:mi>max</c:mi></c:msub></c:math> distributions up to energies of 100 EeV (<e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msup><e:mn>10</e:mn><e:mn>20</e:mn></e:msup><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mi>eV</e:mi></e:math>), not yet revealed by current measurements, providing new insights into the mass composition of cosmic rays at extreme energies. Gaining a 10-fold increase in statistics compared to the fluorescence detector data, we find evidence that the rate of change of the average <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:msub><g:mi>X</g:mi><g:mi>max</g:mi></g:msub></g:math> with the logarithm of energy features three breaks at <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mrow><i:mn>6.5</i:mn><i:mo>±</i:mo><i:mn>0.6</i:mn><i:mo stretchy="false">(</i:mo><i:mrow><i:mi>stat</i:mi></i:mrow><i:mo stretchy="false">)</i:mo><i:mo>±</i:mo><i:mn>1</i:mn><i:mo stretchy="false">(</i:mo><i:mrow><i:mi>syst</i:mi></i:mrow><i:mo stretchy="false">)</i:mo><i:mtext> </i:mtext><i:mtext> </i:mtext><i:mi>EeV</i:mi></i:mrow></i:math>, <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mn>11</o:mn><o:mo>±</o:mo><o:mn>2</o:mn><o:mo stretchy="false">(</o:mo><o:mrow><o:mi>stat</o:mi></o:mrow><o:mo stretchy="false">)</o:mo><o:mo>±</o:mo><o:mn>1</o:mn><o:mo stretchy="false">(</o:mo><o:mrow><o:mi>syst</o:mi></o:mrow><o:mo stretchy="false">)</o:mo><o:mtext> </o:mtext><o:mtext> </o:mtext><o:mi>EeV</o:mi></o:math>, and <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:mn>31</u:mn><u:mo>±</u:mo><u:mn>5</u:mn><u:mo stretchy="false">(</u:mo><u:mrow><u:mi>stat</u:mi></u:mrow><u:mo stretchy="false">)</u:mo><u:mo>±</u:mo><u:mn>3</u:mn><u:mo stretchy="false">(</u:mo><u:mrow><u:mi>syst</u:mi></u:mrow><u:mo stretchy="false">)</u:mo><u:mtext> </u:mtext><u:mtext> </u:mtext><u:mi>EeV</u:mi></u:math>, in the vicinity to the three prominent features (ankle, instep, suppression) of the cosmic-ray flux. The energy evolution of the mean and standard deviation of the measured <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"><ab:msub><ab:mi>X</ab:mi><ab:mi>max</ab:mi></ab:msub></ab:math> distributions indicates that the mass composition becomes increasingly heavier and purer, thus being incompatible with a large fraction of light nuclei between 50 and 100 EeV. Published by the American Physical Society 2025