SIOUX FALLS, SD - It’s a tense situation alongside the river that flows through Falls Park in South Dakota as rescue crews work to locate two men who disappeared while trying to save a 4 year old boy who had slipped.

Making the search particularly difficult are the swift currents and those mounds of eerie foam all down  river from a waterfall for which the park is named.

The Big Sioux River is covered with foam because the churning ice particles mixed with chemicals in the water.

Witnesses say the boy was looking at the river when he slipped and fell in Thursday evening.

Two men grabbed the boy, but then disappeared under the foam.

Sioux Falls, South Dakota police searched the river for the men overnight, but they are believed to be trapped in the foam.