The Nine Hells:
Abandon hope, all ye who enter here.
From the new back to the old. The Nine Hells of Baator have been around pretty much since the earliest days of D&D; the iconic Lawful Evil plane in the Great Wheel, they were basically an expy of Christian Hell as portrayed in Dante's Inferno.
In the World Axis, Baator takes the form of a tormented, burning planetoid roughly 7,000 miles in diameter. Avernus, the so-called "First Hell", is made of the planetoid's surface; the other Hells are continent-sized caverns that lie increasingly deeper within the planet's matter, until one reaches the Ninth Hell of Nessus at Baator's very center. The color veil of Baator appears as a roiling cloud of red smoke, which occasionally parts to permit glimpses of the burning desert below. As mentioned back in the subchapter "Exploring the Astral Sea", any vehicle or individual who preaches the color veil at any point other than the permanent storm that churns above the Lake of Despond, birthplace of the Styx, immediately falls foul of gravity; rather than being transported into a "safe landing" as with the other dominions we've covered, if you can't fly under your own power in normal gravity, you fall - or, more accurately, are sucked down by a fiery, buffetting whirlwind. This is even covered as a unique planar trait!
Plummet to Avernus: Creatures who fall through the dominion’s color veil anywhere other than the storm clouds above the Lake of Despond take 5d10 falling damage, are knocked prone, and take ongoing 10 fire damage (save ends).
Weirdly, it's noted that most creatures survive this descent, which is theorized as being a result of the plane itself not wanting to let them off too easily. Travelers who come by way of the Lake of Despond either descend into the waters (if they're in a vessel), or drop painfully but alive onto the stony shore.
Getting out is harder. Unless you can fly, your only hope is magic, though rumors believe that the Styx ultimately pours back out into the Astral Sea after it passes through Cania.