Running between Ciudad Juarez in Mexico and El Paso in Texas, which sit next to each other on either side of the border, the 300m tunnel was concealed in a storm sewer system and only discovered last week - despite official estimates it took at least a year to build.

Investigators are now looking into whether local officials knew of its construction.

Security has been ramped up on both sides of the border ahead of the inauguration of Donald Trump, who has vowed to launch mass deportations of illegal immigrants once in office.