does this mean descent below the level covered by radar between two pings? that seems very fast to me.
Antiquated idea of how "radar" works these days. What controllers see on their screens is a transmission from the aircraft (transponder, or as Airbubba says, now ADS-B). A flip of an on/off switch in the cockpit
can make a plane "disappear" instantaneously. As can anything that cuts off the transmission: loss of electrical power, loss of airframe continuity. The aircraft does not have to "descend below radar coverage."
The old-school, bouncing radio waves off aluminum,
primary radar is available if desired, but if the aircraft is now a descending shower of confetti, won't help much after the fact. It will take some time to check the primary returns on tape to see what shows up there.
All we really know right now is that the aircraft abruptly ceased transmitting while still at 37000 feet.