The superdeep North Caspian, South Caspian, and Barents basins have their sedimentary fill much thicker and the Moho, correspondingly, much deeper than it is required for crustal subsidence by lithospheric stretching. In the absence of large gravity anomalies, this crustal structure indicates the presence under the Moho of a thick layer of eclogite which is denser than mantle peridotite. Crustal subsidence in the basins can be explained by high-grade metamorphism of mafic lower crust. The basins produced by lithospheric stretching normally subside for the first ∼100 myr of their history, while at least half of the subsidence in the three basins occurred after that period, which is another evidence against the stretching formation mechanism. According to the seismic reflection profiling data, stretching can be responsible for only a minor part of the subsidence in the Caspian and Barents basins. As for the South Caspian basin, there has been a large recent subsidence event in a setting of compression. Therefore, eclogitization appears to be a realistic mechanism of crustal subsidence in superdeep basins.