Those of you of a certain vintage will remember this American sci-fi series from 1966-67
"Two American scientists are lost in the swirling maze of past and future ages, during the first experiments on America's greatest and most secret project, the Time Tunnel. Tony Newman and Doug Phillips now tumble helplessly toward a new fantastic adventure, somewhere along the infinite corridors of time."
I was only seven when it was first shown but I remember being addicted to the show.
By luck (or lack thereof) the travellers, Tony and Doug, frequently found themselves thrown onto the precipice of major historical events: on board the Titanic before it hits the iceberg, in Pearl harbour before the Japanese attack, on Krakatoa before it erupts, and so forth. They would try to warn people about the event, or try to prevent it from happening, while the Time Tunnel crew (led by two scientists and a military general), who once gaining a "fix" can view through the Tunnel the action taking place in the different time, would try to rescue the travelers before the historical calamity befell them too. Episodes always ended in a cliffhanger preview of the next episode. The final episode provided no resolution, as the series was initially scheduled to continue into a second season.
There have been several attempts to revive the format. A pilot was done in 2002 and recently plans for another pilot were unveiled.