Drug kingpin Joaquin "El Chapo" Guzman escaped through a 1.5km long tunnel that was dug under the shower space of his prison cell in central Mexico, authorities say.
After security cameras lost sight of Guzman late on Saturday (local time) guards went into the cell and found a hole 10 metres deep with a ladder, said National Security Commissioner Monte Alejandro Rubido on Sunday.
The hole led to the huge tunnel with a ventilation and light system, Rubido said, adding that its exit was in a building that was under construction.
A motorcycle on a rail system was found in the tunnel and is believed to have been used to transport tools and remove earth from the space, which was 1.7 metres high and around 80cm wide.
Rubido said 18 prison guards will be interrogated by prosecutors in Mexico City.
Until Guzman escaped, Rubido said, "the day had gone on normally and at around 8pm he was given his daily dose of medicine."
The maximum-security prison is 90km west of the capital in Mexico State. Guzman had been jailed there since February 2014.
It is the second time in 14 years that Guzman has broken out of prison. The last time, in 2001, he hid in a laundry basket to escape from a prison in western Jalisco state.
Marines had recaptured him in February 2014 in a pre-dawn raid in a condo in Mazatlan, a Pacific resort in Sinaloa state, with the help of the US Drug Enforcement Administration.
The National Security Commission said on Sunday that an operation to locate Guzman was launched in the area and on roads of neighbouring states, adding that flights were suspended at the nearby Toluca airport.
Soldiers manned checkpoints at a toll booth of the Mexico City-Toluca highway, using lights to look at the faces of car passengers and searching car trunks and the backs of trucks.
The Altiplano prison houses the country's most notorious drug lords, murderers and kidnappers.
Guzman's second escape is sure to embarrass the administration of President Enrique Pena Nieto who won praise for capturing the powerful kingpin, a diminutive but feared man whose nickname means "Shorty."
After his last capture, the government had paraded Guzman in front of television cameras, showing the mustachioed mafia boss being frogmarched by two marines before taking him to prison on a helicopter.
Guzman, 58, whose Sinaloa cartel shipped narcotics across the globe, was considered the world's most wanted drug lord before his arrest.