Ordinary seawater has around 3.5% salt dissolved in it. The Dead Sea has around 34% salt - a massive difference. The salt makes the water so dense and heavy that your body is pushed upward as soon as you step in. You can sit back, lift your feet off the ground, and float completely still without any effort at all, even reading a book or a newspaper. Many visitors find this the strangest and most magical feeling.
The Dead Sea sits in a deep valley called the Great Rift Valley, which stretches all the way from eastern Africa up through the Middle East. Because it is the lowest spot on land, there is nowhere for the water to drain out - so it just stays and gets saltier as water evaporates into the warm air. The surface shimmers in a haze of heat for most of the year.
The muddy shores of the Dead Sea are famous too. The dark mud is full of minerals, and people have smeared it on their skin as a beauty treatment for thousands of years - even ancient Egyptian kings and queens were said to love it. Tourists still scoop the mud out of the shallows and cover themselves from head to toe, then rinse off in the gleaming salty water.