SAN DIEGO (Border Report) — The latest drug tunnel discovered in the San Diego-Tijuana area was No. 91 since 1993, promoting some to ask why there’s been so many.

Noted geologist Pat Abbott said the answer is simple: it’s the soil in Otay Mesa.

The “Otay Mesa area, the border on both sides, is an ideal place geologically speaking for making tunnels,” Abbott told Border Report.

Abbott said the type of sediment found in this area is due to volcanic activity that took place millions of years ago, something hard to find just about anywhere else along the southern border.

Pat Abbott is a geology expert. (Courtesy: San Diego State University)

“When those things were erupting 13 million or 15 million years ago, during that time volcanoes blew ash into the air, that volcanic ash filtered into the sand and gravel in San Diego,” he said. “That volcanic ash buried beneath the surface this converting into clay and the clay gives cohesion to the sediment and that’s what allows the walls of the tunnel to stand up and not collapse.”

And Abbott says if you go just a few miles along the border to the west or the east, the ideal conditions for digging tunnels disappear.

“If you went out closer to the beach along the border, we have younger sedimentary rock and those don’t have the same amount of clay in them and they’re so sandy the tunnels would collapse.”

To the east, where Otay Mountain sits, it’s a different story, but just as hard to set up a tunnel.

“That’s very hard volcanic rock,” Abbott said. “That would be very difficult to dig tunnels through, so there’s a sweet spot and that sweet spot happens to be right in Otay Mesa.”

Drug cartels have apparently learned this, and that is a big reason why they keep making tunnels to smuggle drugs and humans across the border.

The tunnel discovered last weekend was well over 1,700 feet long, 61 feet deep with a diameter of 4 feet at its widest point, according to the U.S. Attorney’s Office in San Diego.

And like many of the others, it had a ventilation and lighting system, its walls were shored up with rails to push carts loaded with cargo.

The south entrance was found below a house on the south side of the border; its northern point was in a warehouse about 200 yards into the U.S.

Abbott said the conditions are so ideal, you could literally dig the tunnel using picks and shovels.

“It’s not incredibly difficult to dig through,” he said.