Agree with Bernadette that this is most likely a hacker / spammer taking advantage of a configuration issue with your website. If you're using a CMS (Wordpress/Joomla/Drupal etc.) make sure that it has been properly configured (or have your website developer do it).
I had a similar instance with a website I inherited a few years back where there was a configuration issue on the CMS that enabled individuals to set themselves up as users and a blogging extension, which had an out of the box configuration issue enabling anyone to create blog posts. Whilst the blogging tool was set to require admin approval to make the article live and visible on the site, once the article was created, it was still somehow able to be indexed by Google which created one hell of a mess.
Fixing the issue in the CMS/Blogging extension was quite simple but the cleanup took a long while and over a period of months I had to disavow a continuing stream of junk links and spent a lot of time writing to other webmasters advising them of the issue with their site so they could remove. Nearly 3 years down the line I still get a few of these pop up from time to time, as there are obviously other sites that have not plugged the gap and updated their blogging tool and as such contain this massive list of dodgy links from link spammers.
If you are using a CMS I would recommend that you, or your webmaster, check the list of authorised users and, if there are any that you do not recognise or you did not create then block them; and immediately take a look at your CMS security settings to ensure that all new users require Admins to approve/activate them before they can do anything.
Unfortunately with this stuff, once the exploits are discovered it is quickly disseminated across the internet and every link spammer (and his dog) tend to jump on-board, so the quicker you can plug the leak and commence remediation the better. Good luck