Just an air pocket! I have hit some dire turbulence over the Indian Ocean. The ITCZ is quite something to behold. But you have no sense of what is happening in the cabin. I have had cabin crew asking me 'how far did we fall?'. It seems like an anticlimax to have to say 'I'm sorry, but we didn't! The altimeter barely deviated.' They insist we must have fallen thousands of feet. You get the sensation from being thrown upwards in a bump, not downwards. The resulting lower 'g' sensation for several seconds makes you think you are falling down. Throughout, you are probably staying within feet of where you should be. I don't think I have ever seen an altitude discrepancy of more than 100' in the most serious turbulence. I have gone off course 150 miles trying to find a way through the monsoon over the Andaman Sea