The Sunken Hotel is the unofficial name of a major location in Spec Ops: The Line.
The hotel itself makes up a large portion of The Nest, and spans over much of Dubai itself. It features prominently in Chapter 3, being introduced at the end of Chapter 2, where a sandstorm actually buries part of it. Much of the hotel complex to speak of is already underground, although largely intact. It is a stronghold for the 33rd infantry, until Delta Force arrives. Some areas have even been fully converted into military outposts.
In Chapter 3, Delta must make their way through a portion of the hotel, the area that has just been buried. In doing so, they discover that the CIA is also in Dubai.
Successfully navigating the hotel in chapter 3, with no deaths or restarts in the campaign thus far, earns the player the achievement Sierra Hotel.