It's available at the map section of the game, at the top there.
Unlike the usual Mines it is not a mine where your heros only stay until timer is done but fight against the opposing clan.
Those clans currently get chosen via a system of 10+/- ranks above or below you in the event top 100 list.
Duration:24 hours from daily reset
You can get the Emeralds the clan wins right after reset. It's not needed to participate in the clan mine to get rewards.
Daily Max emeralds to be gained is 500.
An attack or defense team consists of 8 heroes, where you only can send 4 at once, by spending a horseshoe to get them transported to the mine.
Horseshoes will be refilled for free by the game at reset time each day, if you craft them it will only refill everything up to 10, or if over that number not at all
You find the new horseshoes and the last emeralds shard reward in your mailbox (registry "new" or "system")
Think about the other clan also getting ready to grab the mine. Don't waste the horseshoes by sending a basic hero team just to capture it, it might be the one, deciding on keeping the defense later.
The game provides every clan member a set of 10 horseshoes a day, so planning and participating in clan attacks or defenses is the smartest move, to make the most of it.
Make sure to look into clan chat whenever you have a few moments to spare,
check the clan mine for left defenders and maybe prop it up with some buffers or a mixed team (hitters plus buffers&defenders)
It will be better for the clan and your own progress if you participate, because Emeralds is everything you hunt even after reaching Siege heroes, all level progress later on will be partly based on getting more shards, so get into it-now!
No Clan expects you to send best heroes (score boost!) but in fight situations, remember you can always pull out the team again (disband button in defense list).
Weigh your progress first,
after all its 500 emeralds only for each clan member
It is now safe to do daily boss after doing Dark Ritual.
The guys in the mine will survive and stay in mine unless the die.
They will become available to revive for 10 bottles once they die, or to rehire if you did DR.
It is recommended to play champions league.
It will let you learn the skills of the non siege heros much better, you will know, which debuff hero might fill up your last team, when you don't want to make a DR, as well make you aware of the unique damage types, the best combinations and which buffers work for whom, on the fly - in a little time span you do it without thinking.
Guide by a former Goon