A time-reversal mirror (TRM) was implemented in an experiment conducted in the Mediterranean Sea in April 1996. The experiment utilized a vertical source--receiver array (SRA) spanning 77 m of a 125-m water column with 20 sources and receivers and a single source/receiver transponder (SRT) colocated in range with another vertical receive array (VRA) of 46 elements spanning 90 m of a 145-m water column located 6.3 km from the SRA. The TRM demonstration consisted of transmitting a 50-ms pulse from the SRT to the SRA, digitizing the received signal, and retransmitting the time-reversed signals from all the sources of the SRA. The retransmitted signal was then received at the VRA. An assortment of runs were made to examine the structure of the focal point region and the temporal stability of the process. The process was extremely robust and stable. Iterative enhancement of the focusing process was also demonstrated by a ``ping-pong'' exchange of a pulse between the transponder and the TRM.