Imperial Policy on the Underdark

Since long before the founding of the Empire of Turelion, the presence of the myriad tunnels and caverns that stretch on endlessly beneath the surface of Kelbonnar, has been known, though for most the extent of their knowledge stretches only to the folkloric tales and myths, summarised by academics under the umbrella name of Death’s Endless Passages.
However, the true extent of the Underdark, along with the knowledge of the creatures and humanoids that dwell there is known to far fewer people. It has long been Imperial policy that detailed information on the Underdark is kept on a purely need to know basis and the Imperial authorities have even gone so far as to propagate the tales encompassed in Death’s Endless Passages, to fuel the fear of the common people towards subterranean spaces, with the hope of stopping them from becoming too curious. In several cases, those who have publicly become too interested in the border with and the Underdark proper have even been arrested by the Imperial Inquisition on one spurious charge or another, and have disappeared. Even several formerly prominent members of the Imperial College of Arcanists and other academics from Houses of Learning across Kelbonnar have been made to vanish by the Inquisition for asking too many questions.
The reason that the Empire is so concerned about the Underdark and its denizens is believed to be two fold. Firstly, that they are concerned that an increased knowledge of the Underdark’s passages might lead to transportation routes being discovered that are harder for the Imperial authorities to monitor. This is especially concerning when combined with the fear that the Empire’s enemies, Nova Norgantho primarily, along with illegal organisations such as the Kelrik Pirates, the Black Waves Smugglers, the Scarlets and the White Fangs might use the tunnels to facilitate their illegal operations, or to enable them to launch attacks anywhere and more easily sow discontent and revolutionary fervour across the Empire.
Secondly, the Emperor and the Imperial Court want to heavily manage and monitor any contact with the sapient humanoids and their organisations and even nations that exist in the Underdark, and keeping knowledge of their existence from the general populace is deemed to be the best way to do this. Again, the Imperial authorities are concerned that contact with intelligent entities in the Underdark may lead to war inadvertently breaking out between them and the Empire, or worse one of the powers of the Underdark being persuaded to interfere with the surface world and the Empire, whether that be to advance the cause of say the Rebel Nation or a particularly ambitious individual.
Whilst knowledge of the true extent and nature of the Underdark and its occupants is kept as secret as possible, a whole section of the Imperial Inquisition, known as the Inquisitor Pioneers has been created especially to explore the region along with investigating and even making contact with the creatures that live there.
It should be noted that nations and organisations that exist outside of the Empire's jurisdiction and influence have their own, generally more complete knowledge of the Underdark. The Vampires of Ymeris, for example are thought to have strong diplomatic relations with several powers that exist there, whilst the Elves of SparosHeavenspire Mountains.