Downtown LA has tunnels for cars at Second and Third streets that are both well-known and well-traveled, but a series of smaller, underground, pedestrian tunnels under Civic Center—below city buildings such as the LA County Hall of Records and the Kenneth Hahn Hall of Administration and used mostly by workers—remain less traversed.
You might recall that the HBO show True Detective ended up having a tense, ill-lit shootout in the subterranean passageway between those two buildings. KPCC has revisited some of the cooler past uses of these tunnels, including ferrying a billion dollars in the dead of night.
In 1960 there was, of course, no online bill-paying, so it wasn't unheard of for people to pay their property taxes in cash. In October of that year, after everyone in LA county had paid, it amounted to a billion dollars, and that caused a problem for the county. They had just moved into new offices at the Hall of Administration, but all the money from the taxes was at their previous office in the old, now-demolished Hall of Records, about two blocks away, says the LA Times. (It was approximately next to what's now the Clara Shortridge Foltz Criminal Justice Center.)
So the county decided to use the tunnel to the Hall of Administration to transfer all those property taxes into their new offices. The move was planned over the course of three months, and lasted from midnight to 7:30 a.m. "under the protection of sheriff officers armed with gas grenades, shotguns and sub-machine guns," says KPCC. It was a success, and the money arrived safely at the new county offices.
Before that, in 1951, tunnels leading from the federal courthouse to the Hall of Justice were used to transport mobster Mickey Cohen, charged with tax evasion, between his jail cell at the Hall of Justice to his court dates, the LA Times reports. That tunnel was closed in 1994, when the Hall of Justice was shuttered due to damaged it sustained in the 1994 Northridge earthquake. (It's not clear whether or not the underground connection to the HoJ has been reopened, but it seems unlikely.)
The tunnels accessible today run beneath not just the Hall of Records and the Hall of Administration but also the Stanley Mosk Courthouse and the Foltz Criminal Justice Center; KPCC notes that they're very bare-bones and surprisingly warm inside. The Hall of Records tunnel is often considered the easiest to get into, accessible from a weird, standalone elevator behind the Hall of Records building. (Look for the red signs that go on about the rules and hours of the tunnels.) Avoiding Regret has a pictoral guide to getting in and what explorers might see.