Suicide Falls

Suicide Falls is the downward terminus of the Ditch, a waterway in Sigil between the Lower Ward and Hive Ward. This is probably the easiest way to physically exit via the edge of Sigil for those without the power of flight.
Not quite a waterfall, water (and jumpers) that goes over the edge of Sigil simply disappears. As there's no turbulence common to waterfalls, the "falls" are eerily silent. Anyone who has looked over Sigil's edge has reported that outside of Sigil is simply nothing. As such, bodies are never recovered. The Harmonium sometimes say that missing youths must have "succumbed to the Falls' siren."
The terminus is actually a delta, with a collection of small islets dotting the waters. On one of these islets grows the Last Kiss Blossom.

References:

---

Polyhedron Magazine #137, p. 14.
https://planescape.fandom.com/wiki/Suicide_Falls