The Tunable Delay Line (TDL) is the most important building block in a modern cell-based timing circuit such as Phase-Locked Loop (PLL) or Delay-Locked Loop (DLL). In previously proposed TDLs, one dilemma exists -- they cannot be both power efficient and environmentally adaptive at the same time. In this paper, we present an effective solution for such a dilemma - a novel "ping-pong delay line" architecture. The idea is to use two small cell-based delay lines operated in a synergistic manner in the sense that they exchange the "role of command" dynamically like in a ping-pong game, and thereby jointly reacting to severe environmental changes over a very wide range. This proposed ping-pong delay line has been incorporated in a Delay-Locked Loop (DLL) design, to demonstrate its advantages by post-layout simulation.