I feel like water plays a big part.
[SPOILARZ]
The inter-dimensional rip originated in a giant sensory deprivation tank, the monster first shows up in the rain, Barb drops her blood/gets taken at the pool, then The Upside Down flickers into existence when Will goes to wash his hands. The camera actually focuses on the water in the sink, leading me to believe it's important.
Also, my thoughts after finishing the series. Again, spoilers.
There's no doubt in my mind that El is alive, but that raises some questions for me. If Hopper is the only one who knows this, he must be in connection with someone at the Hawkins Lab - surely as a result of the deal he made (dammit, Lando). Given where he left the food, I feel like El is alive in another dimension. Maybe not The Upside Down, but another dimension at least.
My mind is reeling about Will right now. He coughed up an over-sized Khan worm, saw a flicker of The Upside Down, then sat down to Christmas dinner like nothing happened. I feel like there's a "parasite controlling the host" kind of thing going on, especially since no one else noticed the inter-dimensional flicker.
[END SPOILARZ]
Hopper might be my favorite TV character ever. Also, he sounds like Harrison Ford.