The smell of chemical solvents was overwhelming at the remote mountaintop site in the northern state of Sinaloa, where Navy personnel on patrol last week stumbled across an enormous holding tank they initially thought might be used to water a marijuana plantation.

Instead, the tank fed water to a pair of enormous sheds where sailors found 12,905 gallons (49,640 litres) of ephedrine, a chemical used to make methamphetamine. That is enough to produce 40.2 metric tons of the drug, or about 309 million individual doses.

The members of the Navy patrol found drums, barrels and other chemicals used in the process at the site, located on a dirt road miles from the nearest town.

The Navy was carrying out anti-drug operations in the area, Mexico's so-called Golden Triangle, where traffickers long have operated. But was no immediate indication which drug cartel ran the facility.

The Navy also reported on Tuesday that it had detected a shipment of cocaine hidden inside the carcasses of frozen sharks aboard a freight ship at the Gulf coast port of Progreso. The Navy did not provide an immediate estimate of the amount of cocaine found, but said it had been detected in an X-ray inspection of the shipment.

Also on Tuesday, police found the bodies of seven young men who were beaten or shot to death in the state of Durango in northern Mexico. At least three of the bodies had bullet wounds. The others appear to have been beaten to death.

Investigations into the case are continuing, but the style of the killings suggested the involvement of drug gangs.

An employee of the state prosecutor's office, who was not authorized to be quoted by name, said the bodies were found on a street in the city of Gomez Palacio. -- AP