The Deep Sinks is a fascinating feature in the forested wetland portion of the park.

Years ago there were two separate holes but now there seems to be only one, yet the name in plural remains. The holes were probably formed by decomposition of the peat soils, supported by contact with oxygen in the water. Holes like this one are common in older bog wetlands such as the West Hylebos Wetlands Park.

The West Hylebos Wetlands contain peat soils up to 30 feet thick. The peat soils formed some 10,000 to 15,000 years ago in a depression left after the last glaciation. The depression held a lake, which gradually filled with sediment and plant material until it finally became dry enough to support forest growth. In places the organic plant deposits (peat) may be 30 feet thick.  Since peat forms in relatively low oxygen conditions the plant materials are only partly decomposed - plant fragments can be seen from plants that died thousands of years ago. Very few examples of this type of wetland remain in the Puget Sound Region, partly because the peat was mined years ago to use as a soil amendment.

The water level in the hole is probably fed by a shallow aquifer (groundwater) from the Panther Lake area. A deeper aquifer called the Redondo-Milton Channel lies underneath this area, extending from the Redondo area southward toward Milton (hence the name, Redondo-Milton). Several wells in the Redondo-Milton channel are the main source of drinking water for Federal Way.