When a certain area of lithosphere reaches the state of isostasy, it is said to be in isostatic equilibrium.

Gravitational measurements were showing that many areas are not in isostatic equilibrium.

Theoretical considerations imply that a topographic low in isostatic equilibrium would exhibit a slight negative gravitational anomaly.

Thus, when loaded, the lithosphere progressively reaches an isostatic equilibrium, which is the name of the Archimedes principle applied to these geological settings.

In practice, for a given area to remain in isostatic equilibrium, there would always be a limiting factor.

But here tectonic pressures must have produced the subsidence and prevented the immediate re-establishment of isostatic equilibrium.

The crust is now also abnormally thick (about 70km) but gravity measurements indicate that it is in approximate isostatic equilibrium.

But even with a rigid outer Moon gravitational forces would have raised the floor most of the way towards isostatic equilibrium.

Any such deep body of liquid is likely to rise to a level corresponding to isostatic equilibrium, and there would be none of the observed mascons.

Thus, even if, as seems unlikely for a cold Moon, isostatic equilibrium was subsequently achieved there would still be a residual basin.