We made an excursion to Lençois Maranhenses which as seen in the foto taken from a small airplane is a huge natural park of white dunes separated by lakes in the north-east of Brazil.

The lakes exhibit many colors and shapes . They fill during the rain season and then dry out in the dry season between July and November. Strong winds move the dunes against the resistence of incipient vegetation .

Our team consisted from left to right of Dirk, Reginaldo (driver), Maduro (guide), Ascanio, Dadinho, myself, Soares, Buda and Thiago (driver).

To get there we had to cross several rivers and lagoons with two Toyotas. The water was very dark due to large quantities of vegetal debris .

The quicksand typically is formed at the shore of drying lakes. The lagoon we chose to study quicksand was lonely and the tropical sun was gleaming.

Stepping on quicksand is like walking on a waterbed (see movie ) except that when the weight is too large the crust of the soil breaks forming brittle cracks and you sink in (see movie ).

After the crust is broken water comes to the surface and the material becomes very soft like a swamp. The brown color is due to bacteria .

By walking on light plates we managed to step on the quicksand without breaking in and to make measurements of the shear strength as function of depth without breaking the crust.

We discovered that before breaking the crust the viscosity essentially does not depend on depth like in a fluid with hydrostatic pressure (black squares) while after the crust broke and the water came out the viscosity linearly increases with depth as in a soil due to Coulomb friction (red circles).