Rashi: It is Galus. Radak - it is worse than the grave, which is called "She'ol Tachtis" (86:13); "Bor Tachtiyos" refers to many deaths in Galus. Captivity is the worst [punishment mentioned in "whoever deserves to die (on his bed) will die (that way; whoever deserves [death via] the sword (will die) by the sword, by hunger through hunger, la'Shevi (for captivity) la'Shevi" - Yirmeyah 15:2), for captivity includes all of them.

Radak: Galus is compared to dark places, and to the depth of the sea.