From Customs and Border Protection:

The tunnel originates in Tijuana, Baja California, Mexico in an industrial area approximately one-half-mile west of the Otay Mesa Port of Entry. Following the discovery in late August 2019, Mexican law enforcement identified the tunnel entrance and members of the San Diego Tunnel Task Force (SDTTF) began mapping the tunnel from Mexico. Concealed by a small industrial building, the tunnel travels north into the U.S. bending slightly west and extending an astonishing 4,068 ft. from the border, with a total length of 4,309 ft.; over three-quarters of a mile. The next longest tunnel in the U.S., discovered in San Diego in 2014, was 2,966 feet long.

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The tunnel, that is approximately five and a half feet tall and two feet wide, has an average depth of 70 ft. from the surface. It includes an extensive rail/cart system, forced air ventilation, high voltage electrical cables and panels, an elevator at the tunnel entrance, and a complex drainage system.

An offshoot from the main tunnel was discovered at approximately 3,529 ft. into the U.S. This offshoot traveled several feet then came to an end without breaching the surface. The main tunnel extended another city block at which point agents discovered several hundred sand bags blocking the suspected former exit of the tunnel in the Otay Mesa warehouse district within the U.S.