Why doesn't a lake or pond simply sink into the earth like a glass of water on your front lawn?
There are several reasons why a lake or pond doesn't just sink into the earth like a glass of water on your front lawn. One reason is the heat of the sun, which causes water to evaporate from lakes and ponds [1]. The second reason is that the water in lakes and large water bodies is huge compared to what is in a beaker or tub, so the process of evaporation is slow [1]. The water that ponds and lakes lose via evaporation is naturally deposited back in them through rain and other water access points [2]. Also, the ground on which a lake sits has a saturation limit, meaning that it can't consume or hold any more of a certain thing, like water [3]. Some lake bottoms are leakier than others, and some lose water due to seepage [4]. They lose more or less water depending on what kind of rock and sediment is beneath the lake [4].
 lakes and ponds do not sink into the ground because the heat from the sun causes water, especially in large bodies, to evaporate slowly. [1] Evaporation transfers water from lakes and ponds into the atmosphere as vapor, which then returns to the surface as precipitation. [1] Since precipitation usually occurs in the form of rain or snow, it replenishes the water in lakes and ponds. [1] Soil also has a saturation limit, and, generally speaking, soil particles are large enough that they cannot absorb additional water. [2, 3] Therefore, while water can seep through the ground, it will eventually become saturated and cannot absorb any more water. [2, 3]