The party is walking in a town district with a lot of wealthy inhabitants. As they walk along the street, they (only 1 person will probably do this) step on a sewer cover, which breaks, plunging the unfortunate PC into the depths below. The actual distance he falls is 250 feet, or, in the case of a munchkin party (i.e., that isn't enough), the distance is 10' more than the farthest infravision of the party. No magic is detectable. The sewer is really a trap by cunning thieves hoping to catch a rich merchant after a large bank withdrawal. From here, the party should go down and help the PC; they can discover a tunnel down there, and they start the typical dungeon crawl.