Another rare exploration site that you might come across is a Sleeper Cache - previously cloaked hideaways used by the Sleepers to store items collected from across known space. They can be very difficult to scan down and, in addition to relic and data vaults, feature Sleeper defenses that must be navigated to reach the loot. Expect to find corrosive damaging clouds, defense turrets, and even minefields barring your progress.
Successfully completing a Sleeper Cache can potentially reward several hundred million ISK of rare loot. They are also the only location for finding blueprints to produce extremely powerful polarized weaponry.