The Moon's Sea of Crises is a dark area near the north-eastern limb, lunar coordinates ca. 20°N 60°E. It has a distinctive oval shape and its dimensions are approximately 270x180km (170x110 mi), the north-south axis being the elongated one. In terms of area covered that makes it about the size of Ireland. It's easy to spot because it's clearly detached from the main cluster of Seas and is the first major feature to become visible following the new moon.
The elliptical crater was, like most other large lunar features, an impact basin. The Mare Crisium though, is an earlier one from when the moon was still "hot" and therefore filled with lava after the impact. Being that old, it also has some sizable craters within it, most prominently the 2000m deep Picard Crater, the smaller Peirce Crater and the strange Proclus Crater near the western edge. It's been a subject of particular study due to its odd geological (selenological?) formations, including what are believed to be glass domes, and illusions of weather phenomena. Underneath its surface lies an enormous concentration of dense rock (mascon) that also makes it a gravitational anomaly. An interesting place indeed.
The Mare Crisium was the landing site of three lunar probes of the Soviet
Luna series (15, 23 and 24), the first two of which crashed. The final mission of the series, Luna 24, successfully soft landed on 1976-08-18 and returned to earth with rock samples.
It's pretty unlikely that there is such a thing as a Russian "Cosmosphere" base near the western rim of the Mare as has been suggested by more imaginative minds. I do believe Siberia was much more conveniently located for the installation of bridges, bases and other technological amenities in the middle of nowhere and for a fraction of the cost. The idea was put forward by famed ufologist and all-out mad scientist Dr. Peter Beter in the late 1970s.
Sources:
Inconstant Moon
Lunascan Project
NASA
NASDA (Japan)