The largest single group of asylum seekers ever to cross into the U.S. tunneled beneath the border wall near San Luis, Arizona, on Monday, voluntarily turning themselves into Customs and Border Protection, according to the agency.

Migrants can be seen marching toward Border Patrol agents by the hundreds, according to video obtained by ABC News. Smugglers dug a series of seven holes, only a few feet long beneath the steel border fence, with hundreds going beneath the wall and a smaller number clambering over it, according to CBP.

The agency says 179 of the record 376 people who crossed were children, including over 30 unaccompanied minors — children under 18 traveling on their own.