General words:
* underestimate
* understand
* underneath
* underwear
* underground
* underdeveloped
* undergraduate
* underemployed
* undernourished
* underwater
* underneath
* undermine
* undertake
* undergo
* undercover
* understated
* underestimate
* undersized
* underprivileged
* understatement
* undervalued
* underage
* underwear
More specific words:
* undersecretary
* undergraduate
* undergarment
* undertone
* undercurrent
* underpass
* underworld
* underbelly
* underdog
* underpin
* underwrite
* underline
* underscore
* underdeveloped
This is just a small sample, and there are many more words out there that start with "under." If you're interested in learning more, you can try looking up words in a dictionary or online thesaurus!