Sleeper Caches are similar to Relic & Data Sites, but they require both a Data Analyzer and a Relic Analyzer. They are difficult to scan down completely, without high scanning skills/bonuses. They contain valuable loot in hackable cans, with some NPC Sentry Guns, invisible minefields, and explosive/corrosive clouds.
These sites come in three variants:
Explorers should have high hacking and archeology skills and bonused ships with a self-repairing tank before attempting these sites. These sites often include multiple “rooms” with hidden rooms and objects and will test and explorers skills. Storyline BPCs and sleeper “blue” loot worth many tens (or hundreds?) of millions of ISK sometimes drop here even in high-sec, so keep a sharp eye out for them, if you're prepared – these are not for beginners.
These may appear by name to be an intimidating retreat where ships go, and then they don't come back, but in reality, these cosmic indications within EVE Online are actually fairly straightforward. They by default, more difficult versions of the data or relic sites you get normally. There is additional risk involved because of various hazards, but you can get some sizable reward for your trouble should you decide to run one, which after this, I hope to convince you that you should.
I should start off, appropriately, with a list of cautions and advisories:
There are three kinds of Sleeper sites: Limited, Standard, and Superior; in this order is their typical difficulty in the same fashion. Unlike regular data and relic sites, Sleeper site loot is held in storage depots, which are accessed with the Relic Analyzer. Site-related functions, such as switching off sentry guns, turning off toxic gas clouds, resetting timers, and so forth are all data hacks which use the Data Analyzer. While failing certain Data hacks can be decidedly unpleasant, nearly all the Relic cans can be failed repeatedly without ill effect. Superior Sleeper sites do have a different mechanic in the Archive Room.