Mare Insularum is a lunar mare

Lunar mare

The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...

 located in the Insularum basin just south of Mare Imbrium

Mare Imbrium

Mare Imbrium, Latin for "Sea of Showers" or "Sea of Rains", is a vast lunar mare filling a basin on Earth's Moon and one of the larger craters in the Solar System. Mare Imbrium was created when lava flooded the giant crater formed when a very large object hit the Moon long ago...

. The basin material is of the Lower Imbrian

Lower Imbrian

In the lunar geologic timescale, the Early Imbrian epoch occurred between 3850 million years ago to about 3800 million years ago. It overlaps the end of the Late Heavy Bombardment of the inner solar system. The impact which created the huge Mare Imbrium basin occurred at the start of the epoch...

 epoch

Epoch (geology)

An epoch is a subdivision of the geologic timescale based on rock layering. In order, the higher subdivisions are periods, eras and eons. We are currently living in the Holocene epoch...

, with the mare material of the Upper Imbrian

Upper Imbrian

In the Lunar geologic timescale, the Late Imbrian epoch occurred between 3800 million years ago to about 3200 million years ago. It was the epoch during which the mantle below the lunar basins partially melted and filled them with basalt...

 epoch. The mare is bordered by the crater

Impact crater

In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

s Copernicus

Copernicus (lunar crater)

Copernicus is a prominent lunar impact crater named after the astronomer Nicolaus Copernicus, located in eastern Oceanus Procellarum. It is estimated to be about 800 million years old, and typifies craters that formed during the Copernican period in that it has a prominent ray system.-...

 on the east, and Kepler

Kepler (lunar crater)

Kepler is a lunar impact crater that lies between the Oceanus Procellarum to the west and Mare Insularum in the east. To the southeast is the crater Encke....

 on the west. Oceanus Procellarum

Oceanus Procellarum

Oceanus Procellarum is a vast lunar mare on the western edge of the near side of the Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather...

 joins the mare to the southwest.

Copernicus is one of the most noticeable craters on the Moon. The rays from both Kepler and Copernicus protrude into the mare. It is located near the crater Fra Mauro

Fra Mauro (crater)

Fra Mauro is the worn remnant of a walled lunar plain. It is part of the surrounding Fra Mauro formation, being located to the northeast of Mare Cognitum and southeast of Mare Insularum...

.

Sinus Aestuum

Sinus Aestuum

Sinus Aestuum forms a northeastern extension to Mare Insularum. It has selenographic coordinates 10.9° N, 8.8° W, and it lies within a diameter of 290 km....

forms a northeastern extension to the mare.