A 21-year-old woman complains of short-term pain attacks in her lower left tooth that are provoked by thermal stimuli and quickly stop after the stimulus is removed. On the masticatory surface of tooth 47, examination reveals a carious cavity with a narrow entrance that is filled with light softened dentin. Probing of the cavity floor and percussion of tooth 47 are painless. Probing of the cavity walls is painful. Make the diagnosis: