You can unlock a Cheat called 'Area 51 Mode' then enable it in the secrets menu! This will change the Deadwings into ufos!
Go to Deadside: Marrow Gates. At the beginning of the level, go to the left and climb up the blood waterfall. Open up the Shadow level 10 Coffin Gate (120 Dark Souls). Along with this cheat you will also get the Book of Shadows.
N64 and Dreamcast Locations:
Not on these versions.
Whether it's day or night in Salvage Yard, Mojave Desert, you can find this secret with or without the L'Eclipser. After making your way through the Salvage Yard, you'll come across an entryway into an underground area. Once inside, you'll notice a plaque behind the central pillar with the words "They are watching us" scrawled on it. Shoot at the plaque to open up a path into a hidden area with a mysterious light at the end of it. Step into it and... you end up inside a dome followed by the "Secret Activated" message.