Police said a decayed skeleton was unearthed in the mud.
The discovery was made in Sheepwash Lane, West Wittering, near Chichester.
The force received a call about the incident just before 5pm.
Sussex Police confirmed the remains were human and an investigation has been launched.
A spokesman said specialist forensic teams recovered the bones before they were sent for examination.
“The discovery is currently being treated as unexplained and enquiries to identify the bones are ongoing,” the spokesman added.
He added that the bones were found in an area where historical burial sites are located.
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