I've seen a number of hidden underground base entry builds over the years, but a lot of them take up tons of space or a lot of resources. I've been working on refining a sticky block version since the first snapshots of 1.8 came out.

This hidden underground base entry is relatively light on resources with 4 sticky blocks, 5 sticky pistons, 12 redstone repeaters, 12 redstone comparators, less than a stack of redstone, and a simple hopper-dropper t-flip-flop. You'll also need a handful of immovable blocks, like obsidian or furnaces, to help make it all work.

Some key features of the build; A full 4-block drop floor that will allow for (3) 3-block tall walkways radiating from the entry, so you can take your horse down into the base with you; supports a variety of hidden activators (check out docm77's hidden base entry with the block drop activator; https://www.youtube.com/watch?v=4aixNDaOhSM); can use in any biome - sand and gravel just need to have signs on the framework to hold the sand up and it won't effect your headroom.