Borealis Planitia is a large plain on Mercury with a smooth floor, thought to be similar to a

lunar mare The lunar maria ( ; mare ) are large, dark, basaltic plains on Earth's Moon, formed by lava flowing into ancient impact basins. They are less reflective than the "highlands" as a result of their iron-rich composition, and hence appear dark to ...

.Topographic Features and Surface History
Atlas of Mercury (NASA SP-423), 1976 It is centered at 73.4° N, 79.5° W. The name is Latin for ''Northern Plain''.

References

Impact craters on Mercury {{Mercury-planet-stub