UPS workers pull parcels across a floor containing casters after they were unloaded from a plane at Worldport, a massive sorting facility in Louisville, Ky.

LOUISVILLE, Ky. — The humming is constant; a low-pitched drone from 155 miles of conveyer belts racing packages in every direction. Boxes shift from one belt to another and bump into a metal wall. Thud. Thud. Thud. In the background, trucks beep and jet engines roar.

Forget jingling bells and ho-ho-hos, these are now the sounds of the holidays.

As more gift-givers shop online, there are more packages to ship. Online sales now account for 10 percent of all shopping and 15 percent during the holidays, according to research firm Forrester. That leaves FedEx and UPS with a combined 947 million packages to deliver between Black Friday and Christmas Eve — up 8 percent from last holiday season's forecasts.

For UPS, the key to getting all those last-second orders delivered on time is Worldport, a massive sorting facility located between the Louisville airport's two main runways. On a typical night, 1.6 million packages pass through. Just before Christmas, there can be 4 million, peaking tonight.

UPS plans to deliver about 36 million packages on Tuesday, its busiest day of the year, up from 35 million last year. That includes all of Worldport's shipments plus those traveling by truck.

Standing next to the runways just after midnight, jet headlights can be seen lined up miles away. Every 60 seconds another plane lands on one of the two parallel runways and pulls up to the facility — the size of 90 football fields — to unload its goods.