This study reveals a previously unreported phenomenon: elastic softening in synthetic diamonds at temperatures below 1 K. We present ultrasonic measurements on single-crystalline, non-irradiated synthetic diamonds — specifically, type-IIa (colorless) and type-Ib (yellow) samples grown by high-pressure high-temperature (HPHT) synthesis, as well as type-IIa diamonds grown by chemical vapor deposition (CVD). A pronounced, divergent decrease in the elastic stiffness constant C₄₄ was observed in all samples down to 20 mK. We attribute this softening to electric quadrupolar degrees of freedom with an irreducible representation T₂ in the diamonds. However, the microscopic origin of this effect remains unclear. Drawing an analogy to behavior observed in silicon, we suggest the presence of an as-yet-unidentified, defect-derived quantum ground state with T₁ or T₂ symmetry, present at ppb-level concentrations in all three diamonds studied.