In this case, we are interested in: the lunar seas can be located above the general surface level. Lava was thrown out on the already hardened crust, that is, on areas of the lunar surface, which were a petrified mass, similar to the continental plains, but with some differences: there were no ring structures and no mountain structures in these areas.
There are many doubts about this: if, they say, lava flooded the mainland regions, then it could not be so powerful as to flood high mountain structures of several kilometers. These areas were lowered by the time of the lava eruption, which is what some other “seas” not from the central part tell about.
The erupted lava flooding these areas, as it were, raised their level closer to the general level. But how can we be sure that lava eruptions are not repeated several times at the same place? And this can be assumed. The central part of the disk of the visible side from the moment of the displacement of the nucleus is the most active zone. And if, in reality, lava erupted repeatedly, then this itself makes it clear about the buildup of layers. This is one side.
Another side of the influential action on the rise in the level of the formed "seas" is the influence of the Earth's gravitational forces on the lunar core, which raises and bulges the central part of the visible disk.
One more thing. If we consider the solidified lunar shell as a closed vessel containing an incandescent mass of matter. And inside this vessel, pressure constantly arises, exceeding the forces of gravity. Plus, the damaged integrity of the shell in this area. Another plus of the attraction of the lunar core to the Earth. Then all this taken together creates the conditions for the growth of the planetary body one-sidedly - on the visible side. (Expansion on the visible side is evidenced by the oceans, on the opposite - thalassoids.)
And the increase in the thickness of the crust occurs due to the incandescent masses thrown onto the surface of the hardened shell at times of violent internal processes. This is, on the one hand. On the other hand, the cooling of the subcortical inner membranes. But since we are dealing with the lunar surface turned to the Earth, then its thickening occurs due to the first. Therefore, places with a violation of the "tightness" of the lunar crust were at one time accumulators of thickness and the formation of gentle or flat elevations. These are the "seas".
Having carefully examined the images of the lunar surface, one can once again be convinced that lava eruptions have repeatedly appeared in the same place, flooding the space not only of the "seas", but also of the continents, flooding individual mountain structures and entire mountain ranges. But since the color of the continental plains does not coincide with the color of the "seas", such a difference was established. The substance of the seas is much younger and fresher than that of the mainland.
There are also samples of such huge lava eruptions on our planet - these are mountain plateaus.
And one more factor - the main one - the seas were formed due to expansions. This is evidenced by ruptured edge craters in the same seas.