Google's DNS has been working all day here. The problem is that Dyn's DNS server is being DDoS'd; if you request a record that the authoritative DNS server for is hosted by Dyn, then when you query Google's DNS for that record, then Google's server needs to make a query to Dyn, which is down, and thus, your query fails. But queries to Google for non-Dyn domains will continue to work just fine.

OpenDNS works because, as another poster notes, that, for better or worse, they don't strictly obey TTLs: https://news.ycombinator.com/item?id=12762429

Sorry for the confusion about saying Google NS is down :-) I meant dig heroku.com @8.8.8.8 does not work (as pointed by some other poster it is because google NS honors TTL but opendns does not)

Google's OpenDNS servers are not down, they are recursive resolvers, and are not getting data from Dyn's authoritative resolvers. OpenDNS caches DNS data for longer than other recursive DNS providers (they call it smartcache, IIRC), which is why they are still working.

Here's the link that verifies OpenDNS's addresses (if you're wary of trusting a single HN comment): https://use.opendns.com/