Jordan Peele's psychological horror Us focuses on The Tethered and underground tunnels that haunt audiences, but what exactly are they?

Willy Wonka And The Chocolate Factory's tunnel scene is famously terrifying. Here's a list of all the imagery that flashes during the sequence.

In Rambo: Last Blood, there is an expansive network of tunnels underneath Rambo's family farm. Why did he build them and what purpose do they serve?