I've been in the Blue Lagoon and I can attest: It's nice and relaxing and the look is quite unique. As for the marketing the story I heard was that the workers of the powerplant went swimming in the water and found it to be really nice...and then some smart people decided to market it as a spa.<br><br>And yes: The earth would most likely run out of heat eventually...but it will probably be swallowed by the sun before that.,(reply>), @kulka92 The core is definitely molten. That is one of the key differences between Earth and Mars. Mars' core cooled down and solidified, so mars lost its magnetosphere, leaving it largely unprotected from radiation.,(reply>), By which point it should gain plenty of heat back, so it never truly runs out,(reply>), @PerssonTM Yes, but not in the way'd you be expecting,(reply>), If would take 90 billion years for our planets core to completely solidify, but the sun will burn out in a fraction of that time about (5 billion years), anyhow we will probably have left earth by the time any of these events would become dangerous to us.,(reply>), @ZOMBIEBLASTER57 and the very core, apparently.