Phlan has been destroyed and rebuilt dozens of times in its long history, which created haphazard catacombs below it. However, when Abeir Returned over a century ago, those tunnels were replaced by the dragonborn city of Zolgotha. Weirdly though, when Abeir and Toril were finally freed of each other during the Sundering, Zolgotha did not retreat from the Realms.
Today, those catacombs below Phlan are known as Leviathan. Those aware of the network of tunnels just below the city's streets wisely avoid them. They warn that only the truly foolish would brave those darkened hallways of peril and pain. Leviathan is filled with monstrous creatures and wicked defenses that have protected most of it's secrets for over a century.
However, in the last year, the catacombs has been invaded by the Cult of the Dragon, hoping to find treasure and weapons in their newfound goal of freeing the Mother of Dragons, Tiamat.
There are even rumors that the Harpers have have lost agents in the bowels of Leviathan.
The monstrosity that resides below Phlan is a lasting and deadly menace, indeed.