HOLLIS has two main parts. There’s the traditional Library Catalog of Harvard's books, journals, films, sound recordings, maps, and so on. Then there’s a huge additional database of articles, reviews, etc. which come from various sources outside Harvard. Many of these are available online through Harvard's subscriptions.
You can search the Library Catalog separately, or -- especially if you want to see articles -- search for "Everything" at once. These choices appear as soon as you start to type a search.
The Library of Congress's subject heading system uses major terms like Sea level, with added words to specify place, aspect, time period, etc. To find the right subject heading, search for your topic in HOLLIS using the Library Catalog mode, and look for the subjects in your search results. Then you can use them in a Starts With / Browse search to look for more books on the topic. See the example below.
Search limits such as these appear on the right-hand side of the HOLLIS screen when you do a search. Most databases provide similar ways to refine a search.