Many anticipated discoveries in fundamental science demand better measurement sensitivity. For acoustic sensors, mechanical dissipation sets this limit via the fluctuation-dissipation theorem. Yet, even in high-purity crystals, its microscopic origin remains poorly understood, and external enhancement, such as tension-induced dissipation dilution, is difficult to realize. Here, we realize a strain-engineered diamond nanomechanical platform using van der Waals self-assembly that harnesses surface forces to apply tensile stress exceeding 1 GPa. At cryogenic temperatures, these resonators achieve quality factors beyond 10^10. This exceptional coherence allows us to resolve residual dissipation channels, elucidating distinct two-level systems and topological dissipation from a surface superfluid helium film. Advancing mechanical coherence therefore opens access to new regimes of physics in hybrid quantum systems, precision metrology, and condensed-matter physics.
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