In one of the biggest drug busts in U.S. history, U.S. and Mexican agents seized a total of 30 tons of marijuana from both sides of a cross-border tunnel this week. The "sophisticated" Tijuana-to-San Diego passageway had only been in use for about a month, U.S. officials say. Here's a look at the tunnel and the big bust:

Where does the tunnel begin and end?
The tunnel stretches 1,800 feet — the length of six football fields — from a 23,000-sqare-foot warehouse in the industrial Otay Masa area of San Diego to a two-story abandoned warehouse on the outskirts of Tijuana. (Watch a CNN reporter go inside the tunnel)

How "sophisticated" is it?
"There's ventilation, a little bit of a primitive rail system in there, and there's some lighting," says Immigration and Customs Enforcement (ICE) special agent Joe Garcia. It's 20 feet below the ground, four feet high, and three feet wide. Other than that, the feds aren't dishing out many details, says Rosa Golijan in Gizmodo. We know that the tunnel "definitely housed a reasonably high-tech operation," and that, based on the photos released, "it's not pretty."

How unusual is it to find tunnels?
The feds say they have closed 125 cross-border tunnels, at various stages of construction, since the early 1990s, with 75 of those uncovered in the past four years alone. This one is neither the longest — a 2,400-foot-long Tijuana–Otay Mesa tunnel was uncovered in 2006 — nor the most structurally ambitious uncovered so far, but it is notable in that it was fully operational and that drugs were found on both sides of the border. 

How did the feds find it?
The San Diego Tunnel Task Force — made up of federal and California anti-drug and immigration agencies — had the "suspiciously inactive" U.S. warehouse under surveillance, and when a tractor trailer left, they followed it 80 miles north to a checkpoint. The truck had 10 tons of marijuana in the back, and when the feds raided the warehouse they found another 16 tons. The entrance to the tunnel was hidden in the floor of a storage closet. Four tons were found on the Mexican side. The unidentified U.S. citizen driving the truck was arrested along with his Mexican wife.

How much is 30 tons of marijuana?
The stash is equal to about 60,000 lbs. of pot, or 960,000 ounces, or 27,216,000 one-gram joints, and has a street value of about $20 million. "I can promise you there are some very unhappy people in the cartel," says ICE director John Morton.

How does the drug seizure rank?
It's in the Top 5 U.S. busts, says the DEA — probably No. 2, after a 33-ton pot seizure in Oregon in 2008. It looks pretty small, though, compared with Mexico's record capture of 134 tons of marijuana in Tijuana near the end of October. That massive bust was likely tied to the Sinaloa cartel, and it's unclear if the two cases are connected.

Sources: LA Times, San Diego Union-Tribune, Christian Science Monitor, BBC News, Gizmodo, NPR, Time, Opposing Views