Despite the calm surface, there was an undercurrent of dissatisfaction in the room.

undercurrent 由前缀 under-（表示"在...下面"）和 current（水流、气流）组成，字面意思是"潜流、暗流"。引申为"潜在的情绪或倾向"，指表面之下不易察觉但确实存在的情绪或趋势。