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

lunar mare The lunar maria (; singular: mare ) are large, dark, basaltic plains on Earth's Moon, formed by ancient asteroid impacts on the far side on the Moon that triggered volcanic activity on the opposite (near) side. They were dubbed , Latin for 'seas' ...

.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