How are large snails able to climb trees?
The foot of the snail secretes a sticky mucus which allows the snail to climb even the smoothest surfaces. The mucus have two states: one in which it is almost liquid to lubricate motion, and the other - a very viscous one, in which the mucus is sticky and &quot;glues&quot; the snail to the surface. The mucus can alternate between these two states via the pressure the snail activates on it. The gluey state sticks the snail to the surface via adhesion.

Researchers discovered nearly 30 years ago that snail mucus has some unusual properties. It allows the animal to stick to a surface while moving, with the mucus changing its characteristics according to how firmly the snail presses on it.  The slime initially acts like glue, sticking the snail to the surface. But when the snail's foot presses down hard enough on the mucus, it becomes more liquid, allowing it to flow underneath the moving snail. Existing theories assumed that this special characteristic of snail mucus was always necessary for the snail to push off and move forward.

Cited from: https://news.stanford.edu/news/2011/april/snail-slime-trails-040611.html