The thermal water is known for its medicinal properties. Lots of good bacteria, green algae, and other really healthy stuff...well, except for the radioactive material also present (see Wikipedia). all the dots you see in the picture are people's heads. Lots of tourists come from all over Europe to be cured (and even the US....ME!).
They have a really cool system - you get a watch-like bracelet when you pay the entry fee and place it on the turnstyle to unlock it. Then upon entering the locker room you scan it on a reader and up pops a number (in my case 2008) - this is your locker and the scan unlocks it each time. The bracelet is obviously waterproof too.
This is the ditch that outflows water from Lake Heviz and people who swim in it get to save the Forints...but check out the no swimming sign (for 100 meters) so I wonder what the price is for this free dip!