Certain phrases in the scriptures are to be taken literally, and some are to be taken figuratively. The doctrine of 'a lake of fire and brimstone' is a figurative representation of the pangs of guilt suffered by the wicked as they suffer endless torment. There will not be a literal, physical lake into which they are cast.
Joseph Smith
"A man is his own tormenter and his own condemner. Hence the saying, They shall go into the lake that burns with fire and brimstone. The torment of disappointment in the mind of man is as exquisite as a lake burning with fire and brimstone. I say, so is the torment of man." (Teachings of the Prophet Joseph Smith, p.357)