After bitcoin block 15323 was solved, more than 24 hours passed before the next block. This is quite a bit longer than the target time of ten minutes. Looking through the blocks at that time we see that they are mostly all empty, and there isn't much action with the difficulty. Even a full day of the network and - not a single transaction. This was the first year of mining and a lot of people just happened to turn their computers off for a spell.
Fastforward to 2016, now looking at the Woodcoin block chain. In some sense we see a similar phenomenon, some long periods sans blocks. This is a small coin, barely worth more than bitcoin was worth in 2009 at the time of block 15323. However the mechanics here were very different: a real battle over the blockchain. Here's a chart showing some of the action:
You can see that most of the time, the network produces blocks near the target time of 2 minutes, which is 720 blocks per day. However starting in late 2015, and continuing off and on until early 2017, the number of blocks per day drops quite a bit. What was happening?
Drive-by Hashing
What was happening was that unknown miners were dropping a large amount of hashpower on the network, waiting for the difficulty to go up, and then leaving the network. The rest of the miners then were forced to mine at much lower profitability as they struggled to solve blocks at a fraction of the usual rate. For much of this time period a mining rig of 10 or so nvidia GPUs may have been enough to dominate the network and pull off such a disruption.
While waiting longer than usual for your transactions to clear with an anonymous exchange might be annoying, it isn't a disaster. Transactions eventually go through, and are inherently unalterable so shouldn't cause any worry. However some other shenanigans also appeared on the network during this time.
Rewinding the Drive-by
Although it isn't visible in the block chain today (which keeps only the longest chain demonstrating the most skein work), there was in fact another interesting battle occurring on the woodcoin blockchain in 2016. Some might remember that large chain reorganizations occurred during this time as well.
Why the reorganizations? It appears that somebody found a way to battle the drive-by hashers by simply not accepting the rapid blocks they published to the network. Instead, a second group of miners mined in private on the chain of the previous (lower) difficulty. Because the drive-by miner always left and then didn't continue to mine woodcoin blocks, eventually this new party was able to surpass the work of the driveby-hasher's chain and then submit their new longer chain to the network, claiming these blocks for themselves.
This caused some trouble, as pools suddenly saw all their winnings orphaned. They complained and in one case even dropped support for woodcoin. However, this was an amazing test of byzantine consensus and robustness of the woodcoin network using the satoshi protocol. Consider it a stress test.
Could something like this happen today? Well maybe. The hash rate is a lot higher today however:
As the hashrate grows, it becomes harder and harder to play these kind of games in a profitable manner. While this is great for the health of the woodcoin network it's a drag for those of you looking for blockchain battles and associated drama. Well we can't please everyone can we.