Like a lot of deep underground mines, Hera probably started at...

Like a lot of deep underground mines, Hera probably started at surface (probably about 270 or 300m above sea level?) but once you start mining down and get to sea level (0m RL) you then have a choice - let your RL's and levels start to become negative i.e. -50mRL then -100mRL etc etc or as commonly happens, they just add a nominal 10000m to the RLs so that the RL's and Levels never go negative (i.e. start of portal (surface) would now be say 10300mRL and 300m below that is 10000mRL and 300m below that is now 9700mRL etc etc)

There are a number of logistical and computer/engineering/survey/mining/geological model reasons why this happens but you can see why it would be confusing and counter-intuitive to have negative RL numbers as you develop deeper in a mine because they would be incrementing as you go down deeper.

Imagine the VHF radio call to a truck to dump ore in the "Two hundred level decline stockpile cuddy" - question would be "Is that the Two hundred level cuddy or the Minus Two hundred level cuddy?".

Wheras with the instruction to dump ore in the "10100 level cuddy" or the "9700 level cuddy" there isnt much room for confusion or doubt.