DUMB is an acronym that stands for Deep Underground Military Base. The term was first coined by Phil Schneider during a 1995 MUFON presentation. (The presentation is available online at https://www.youtube.com/watch?v=pT1SnoHxxGo).
Schneider claimed he had worked at such an underground facility, and that it was a joint ET / US Military facility, where both Tall and Short Greys were present. He also claimed that meetings were being held where the Tall Greys held an important position.
(Schneider died under mysterious circumstances some months after his presentation).