The Hedera Consensus Service (HCS) gRPC API is a public mirror node managed by Hedera. It offers the ability to subscribe to HCS topics and receive messages for the topic subscribed. API docs for the mirror nodes can be found here:
If you don't have access to a Hedera mirror node, you can check out some of the community hosted mirror node services as alternatives below.
In the future, the Hedera mirror nodes can be hosted by anyone in the community. Currently, the Hedera mirror node software required to run a Hedera mirror node is in beta development. You can check out the code here.
Contributions are welcome. Please see the contributing guide to see how you can get involved.