What is happening? BTC is dropping again? Marketing cap is getting smaller again.
The third bitcoin bubble burst. The first one was in 2011: up from $0.1 in 2010, x300 to $30 in summer 2011, burst and /10 down to $3 half a year later. The second was end of 2013: up from the previous $3 in 2011, x400 all the way up to near $1200 and burst /6 down to about $200 in 2015. The third one is just behind us: up from that $200 in 2015, x100 all the way up to near $20 000. We don't know how low it will go. If history is an indication, down to a few $1000 in a year or two years.
It is normal bubbly behaviour for a pyramid-style asset such as bitcoin. The reason is that bitcoin has been, since its inception, a "get-rich-quickly" pyramid game, where you can acquire coins for not much effort in the beginning, and sell them to greater fools at high ROI later. Each time, you get a larger set of greater fools that buy at the top, and pay for all the benefit of the previous gamblers. These greater fools get burned, and it takes time for them to accept their losses. Until they've sold with losses in the market, and heal their wounds, downwards pressure stops from price rising. When the last army of greater fools has sold out and accepted their losses, however, bitcoin's set for the next set of greater fools, and the speculative game starts again. Rinse and repeat. It will stop when one runs out of greater fools. Potentially, the world's population that has sufficient money to gamble.
If you're convinced there's yet another bubble in the making (it may take a few years: the periods between bubbles get larger), you might make a lot of money by buying in. Not yet (even though it doesn't matter: if after the 2011 bubble, you bought in at $15 to see it fall down to $3, you could still have made a lot of benefit at the $1200 bubble ; and if you bought in at $800 after the $1200 bubble, you might still have made a lot of benefit at the last $20 000 bubble). Note that the next one may take a few years if ever.
If, of course, this has been the last one, you're a dodo, but slightly less than the fools that bought at $18 000 of course