"What... ...IS... ...it?"
"The Caspian Sea Monster" was what U.S. Intelligence analysts dubbed the Ekranoplan - the largest of a class of maritime vehicles that is neither airplane nor hovercraft, but somewhere in-between - soon after they viewed the first satellite imagery in the 1970's.
Ekranoplan is Russian for 'screen craft' or 'skimmer'. These beasts are known as Ground Effect Vehicles (GEV's) because they attain level flight near the surface of the Earth due to a cushion of high-pressure air created by the aerodynamic interaction between the wings and the surface known as 'Ground Effect'. The International Maritime Organization classifies the GEV as a ship. A GEV differs from an aircraft in that it cannot operate without ground effect, therefore its operational ceiling is limited relative to its wingspan.
These craft were originally developed by the Soviet Union as very high-speed military transports, and were based mostly on the shores of the Caspian Sea and Black Sea. The largest had max take-off weight over 540 tons with speeds of up to 500 knots. Because they operate 3 to 20 meters above the surface, these craft are capable of zooming across the ocean beneath the radar, relatively undetected.
About 120 Ekranoplans (A-90 Orlyonok class) were initially planned to enter military service in the Soviet Navy. The figure was later reduced to fewer than thirty vehicles, planned to be deployed mainly for the Black and the Baltic Soviet navies. Ultimately eight Ekranoplans were built with different specifications.
Ekranoplans are capable of making landfall.
The 125-ton A-90 Orlyonok - smaller than it's big brother, the 540-ton Caspian Sea Monster.
A Soviet-era Ekranoplan disgorges a BTR-80 Armored Personnel Carrier
These babies are HUGE.
Here's a modern American prototype relative to a Boeing 747.
The Soviets even had Ekranoplans configured as fast-moving seaborne missile launchpads:
Judging by the tubes relative to the size of the vehicle, I'd say those are capable of launching R-11FM (SS-N-1 Scud-A), SA-10 GRUMBLE, or maybe even SS-12 SCALEBOARD missiles - impressive.
There is no doubt a fleet of these things, so equipped, zooming across the Atlantic toward New York and Washington DC, would have seriously tipped the balance of power represented by our carrier battle groups.
There is a ton of Russian footage of the Caspian Sea Monster on YouTube. It's amazing - have a look.
Here's the Beast comin' at ya armed and loaded for bear: