The bottomless pit is the centre of any sphere. Any sphere in which you are at the centre of, everywhere else is up; Hence it is without bottom.
The Earth, all planets, the sun, the moon, the stars are all spheres.(Revelations 9)1 And the fifth angel sounded his trumpet, and I saw a star which had fallen from heaven to earth; and he was given the key of the shaft of the bottomless pit. 2 He opened the bottomless pit, and smoke went up from the pit, like the smoke of a great furnace; and the sun and the air were darkened by the smoke from the pit. 3 Then out of the smoke came locusts on the earth, and they were given power like the power of scorpions of the earth.