Water megamasers orbiting the accretion disks of nearby supermassive black holes provide the cleanest distance measurements in astronomy: pure geometry, one step, no calibration ladder, no standard candles, no cosmic microwave background. Very long baseline interferometry resolves the maser spots, their positions, velocities, and accelerations map the disk, and the comparison of angular and physical scales yields the distance directly. The Megamaser Cosmology Project applied this to six galaxies (NGC 4258, UGC 3789, NGC 6264, NGC 6323, NGC 5765b, CGCG 074-064) and obtained H0 = 73.9 +/- 3.0 km/s/Mpc with no peculiar-velocity corrections and no ladder dependence at all (Pesce et al. 2020, MCP XIII).
This is the measurement that forecloses the easy exits from the Hubble tension. Every systematic proposed against the distance ladder, Cepheid metallicity, crowding, dust laws, supernova calibration, sample selection, is irrelevant here, yet the geometric one-step answer lands on the ladder value of 73, not the CMB value of 67.4. Within ΛCDM the two numbers describe the same constant, so at least one inference chain must be wrong; the megamasers eliminate the option that the discrepancy lives in the ladder's rungs, pushing the problem into either the CMB-era physics or the model connecting the epochs.
The standing is limited by statistics rather than systematics: 3 km/s/Mpc errors leave the result 2 sigma from Planck, suggestive rather than decisive, and the maser galaxy sample is small with little prospect of rapid growth, since suitable edge-on disk megamasers are rare. Profile-likelihood reanalysis confirms the published value is robust to statistical methodology (arXiv:2502.11998). The megamaser result therefore stands as the independent geometric witness: whatever resolves the Hubble tension must explain why one-step physics in the local volume reads high.