General words:
* underestimate
* understand
* undertake
* underneath
* underwear
* underworld
* undergraduate
* underdeveloped
* undernourished
* underemployed
More specific words:
* underarmour
* undercover
* undercurrent
* underdog
* underfoot
* undergrowth
* underline
* underpass
* underpaid
* underwater
Words with slightly different meanings:
* underneath (preposition)
* underground (adjective)
* undertaking (noun)
This is just a sampling, and there are many other words that start with "under." The prefix "under" typically means "below," "less than," "not enough," or "hidden." You can often figure out the meaning of a word by considering the meaning of the root word and the prefix.