Thyra is like an iceberg, 70% of it is hidden Below. Starting as a series of bunkers and tunnels during the War, these were since then interconnected and now span the whole city and well beyond it.
Some are as lit and airy as the streets above. Some aren't.
Some are filled with trade and laughter. Some aren't.
Some are filled with nice sentient beings. Some really aren't.
And there's more. There are Passages down here. The shards of the Night were thrown in 3 dimensions, so they also went down. Some of the passages are known.