Hey, it's only TV. Sure, it's STAR TREK, the most beloved bla bla, and this is a great one, but it's all relative. What it boils down to is a guy in elf ears grooving with a swatch of pizza-colored shag rug.
There's a kind of THIRD MAN noirishness to the tunnel hunt, and it's creepier than many episodes of what is after all one of the better TV shows. The suspense is actually suspenseful. The peril really feels perilous. As a little kid I think I cried when Spock told me that this hideous creature was as sad, scared and horrified as the people it was eating. This was one of my early lessons in empathy, a lesson reinforced by the EMPATH episode which was, if less thrilling, even more melancholy.
What bothered me when I was five was that this thing, which looks like meat and tomato barf, somehow actually consists more or less of rock. Now that kind of choice might seem visionary, a hippie designer's idea of through-the-looking-glass one-universism, but it might also just smack of the drug era.