Devil's Cavern is the secret level in Dead in the Desert, a Keen 6 mod by Ceilick. It is only accessible once Hydro Corp 2 is completed in either way, so Keen gains access to the dock to board the submarine.
The enemies are the Stomper, the Sand Wraith, the Squib (they function as "hazards" below ledges), and this is where the Lavaworm make their only appearance. There are quite a lot of them hanging around on the ceilings.
This level has no less than 100,000 points in it (v3 beta2). That means 36 Lolly Twistpops, 37 Pink Lemonades, 2 Hotdogs, 6 Rainbow Sherbets and Cake Slices alike, and 14 Ice Cream Tubs (3,600 + 7,400 + 1,000 + 6,000 + 12,000 + 70,000 = 100,000). A huge portion of Water Bottles can be found in here as well: 82 in total. Three 1-UPs are available near the end of the level.