Tar pit

Tar pits were a kind of naturally-occurring sinking hazard found throughout the Realms, made up of petroleum and bitumen, a kind of oil and tar separate from common plant and animal based varieties.

Properties
Tar pits were pools of glistening black liquid, and sticky to the touch. They could be extremely dangerous to those who blundered into them, capable of drowning everything from sheep and goats to raging owlbears and small dragons. Being made of unrefined petroleum, they were extremely noxious and toxic, and could burst into flames or an explosion when exposed to fire or sparks. These pits could be used to harvest petroleum and bitumen despite the danger.

Occurrences
Tar pits usually formed when petroleum welled up from deep underground and reached the surface, forming pools of noxious, stinking black liquid that could reach the size of small lakes. If the oil emerged in swampy terrain, it poisoned the area and became a "dead marsh".

They were especially prevalent in Var, Veldorn, Ulgarth, and the nearby Utter East, in Chult, the Alimir Mountains, and on islets within the Lake of Steam. Tar pits also formed within the Underdark, as the empty space of it interrupted the upward flow of the oil towards the surface. Tunnels and caverns could be home to entire lakes of tar, depending on the geology, and were especially prevalent under the Eastern Shaar, while rivers flowed near underground volcanism.

Tar pits began appearing in Myrloch Vale in the Moonshae Isles amid the supernatural destruction caused by Bhaal in 1346 DR. Likewise, the Helmlands contained a great many tar pits due to the magical disaster of Mystra's death which added to the danger of the region, though many of them disappeared over time.

Bubbling tar pits were a common hazard in Avernus of the Nine Hells.

The former Netherese arcanist Sysquemalyn knew a Hellish spell that summoned up tar from the ground, forming a kind of tar pit.