I also found this: Ok, not a dictionary definition, this from my "English speaking" head: Underneath is less used than under (simply a shorter version) Basically they're all the same word, under is just a shorter version of underneath (through years of the language changing with use) Under is probably a little more flexible, so safest to use. While beneath is still a great word, it can sound out-of-place is CERTAIN sentences: I'm beneath the bed. (odd, like you're using old words deliberately?) I'm under/underneath the bed. (ok) “Beneath the supermarket” would sound a little strange too. It’s now the caviar of the three, slightly more up-market, so use it with care and it’ll be great; don’t just spread it around! Source: http://www.englishforums.com/English/UnderUnderneathAndBeneath/bwd/post.htm

underneath: is often used as a noun/adverb under: is often used as an adverb/adjective I think they're interchangeable in some cases. For your information: Under, below, underneath, beneath Under as a preposition can mean 1 - in a place which is directly below: he hid under the table; the coin rolled under the piano. 2 - less than: she is under thirty; he is under age; it was sold for under $100; he ran the mile in under four minutes. Below as an adverb means lower down: he stood on the hill and looked down into the valley below. As a preposition, it means lower than: the temperature never goes below 25?; if you look below the surface; you shouldn't have hit him below the belt; his marks were considerably below (the) average. Underneath as a preposition means under/beneath: she wore a long woollen cardigan underneath her jacket; he was sheltering underneath a chestnut tree. As an adverb it also means under: he was wearing a thin shirt with nothing underneath. Beneath as an adverb means underneath/below: he looked out of the plane at the mountains beneath. As a preposition it means under: have you looked beneath the cooker? He thinks it is beneath him, he thinks it is too insignificant/too unimportant for him to deal with. Source: http://www.english-test.net/forum/ftopic8129.html

i think it can exchange.we often use "under".