They became the underworld. This suggests lasting destruction. The historical prosperous Tyre is not found in history anymore. Mainland Tyre is not located on survey anymore. Skeptics insist that Tyre still exists today as a populated peninsula.
Skeptics refuse to accept that there was a mainland city of Tyre.
And of course, modern aerial photography show us that there is no mainland city of settlement found that can be identified as Tyre. The prophesy does not speak of the destruction of an island or penisula. It is easier to locate an island by the coastline of a desert country than a mainland city. It is just odd that even the foundation of such city close by an obvious island is not found. The island ceased to exist, as it has become a man made peninsula.