he opened the bottomless pit
The shaft leading to the abyss was now opened allowing whatever was therein to escape.
Jehovah once said to Job: Have the gates of Sheol been opened unto thee? or hast thou seen the doors of the shadow of death? (Job Job 37:17). There are worlds of being and of darkness upon which man has never looked. There is a tenanted abyss of which the demons know, and concerning which they besought the Saviour that He would not send them into it [Luke Luke 8:27-31]. It is a dark and horrible prison, in which many, many strange and evil things are shut up. Satan knows of that world, and would fain bring forth its malignant inhabitants into the earth if he only dared. At last, however, he receives permission to bring them, and the fifth trumpet gives the result.1smoke arose . . . like the smoke of a great furnace
the sun and air were darkened because of the smoke of the pit
Here is another cosmic signthe darkening of the sun. There will be numerous situations which result in the reduction of light from the sun during the Tribulation period. See commentary on Revelation 6:12 and Revelation 8:12.
1 J. A. Seiss, The Apocalypse: Lectures on the Book of Revelation (Grand Rapids, MI: Zondervan Publishing House, 1966), 205.