Mercury is the closest planet to the star Sol at a distance of 0.387 A.U.. It is 3,031 miles in diameter. Its day equals 58.65 Earth days and its year equals 87.97 Earth days. With no atmosphere to moderate its environment, the temperature ranges at its surface are extreme. The side of the planet facing away from the Sun is typically a frigid -300 (F while the side facing the Sun reaches a brutal 870 (F. Mercury's gravity is 0.37 that of Earth.
Atom Blake flew himself to Mercury, where he encountered an army of giant green non-Mercurian alien conquerors, ruled by an American Earthling gangster calling himself "Mighty Adwahl", that had subjugated all of the small buglike telepathic Mercurian people. Atom defeated the conquerors and the Mercurians slew Adwahl.
In April 1942, Johnny Thunder and his Thunderbolt helped the intelligent insect-people of Mercury defend themselves from a giant anteater, and then moved their city to an island where other anteaters could not reach them.  It was later revealed that Johnny Thunder had actually traveled to the Mercury of another dimension, not the native Mercury of the Earth-Two dimension.
In 1947, Wonder Woman flew to Mercury in her Invisible Plane. There she and the Holliday Girls encountered a race of giant women, ruled by Queen Celerita, that had enslaved all of the Mercurian men. With the help of Diana's ally Queen Desira of Venus, this situation was soon corrected.
Years ago, Earthgov placed Multivac, a medical facility, in a stationary orbit above Mercury. The facility nearly met with disaster when it was discovered that Mercury was home to a native life-form composed entirely of agitated plasma. These Mercurians would unknowingly pass through Multivac's protective dome as well as through some of the facility's patients.