There are a lot of entrances into the Undergrowth, most are inside buildings and warehouses on the edge of the middle layer of Nakarta. Most are sealed or heavily guarded by unmarked guards. Not many true entrances are easy to access but those who know where they often slink in and out of these places incredibly easily.
Most entrances are elevators that many daredevils think don't work, trying to explore them or power them through outside sources. It's not often that it happens but people who wander downward are either knocked out and returned to the city or never found again, depending on who finds them.
Finding an actively used entrance often means stumbling upon a puzzle you need to solve to get the elevator open or a person you need to find. Active entrances often change as most buildings extended downward, but many were remodeled when the city started rebuilding over a century ago.