It is literally a mound of mining 'spoil' from the nearby former pit - and it is on fire.
The tip itself is over 100 years old and was abandoned, but then for some mysterious reason, it ignited back in 2006, when flames were seen coming from the top.
Of course, it's not practical to do much but let it burn itself out. However, there must be a lot of coal dust in there to have kept it going until now - seven years later.
Link: Official report into the Hollywood Spoil Fire