Our TV picks for Monday, April 15: Ancient Aliens: Declassified; The Synanon Fix: Did the Cure Become a Cult?; So You Think You Can Dance

Ancient Aliens: Declassified

HIST 8 p.m.

As long as there have been people on Earth, there have been sightings of UFOs. In this four-hour combo episode, experts look at the unidentified objects that have clogged up the skies — not just in the modern age, but for centuries.

The Synanon Fix: Did the Cure Become a Cult?

HBO 9 p.m.

In the premiere of this docuseries, Charles “Chuck” Dederich creates a military-like boot camp for children. When his wife dies, things get more abusive as he quickly remarries and requires all members in relationships to end them and find new partners.

So You Think You Can Dance

KPTV 9 p.m.

With dancing superstardom on the line, it’s all or nothing as the contestants battle it out in various dance challenges in order to survive another week. Who will stave off elimination? And who will succumb to the pressure? Cat Deeley hosts.