The water bomb base, or balloon base, is one of the two most commonly used origami bases, along with the preliminary base, it’s inverse. It gets it’s name from the traditional origami balloon, which uses the water bomb as it’s base.
Once folded, the water bomb base is shaped like a isosceles triangle with a central crease dividing it into to right triangles, and four triangular flaps, two on each side .
create the crease pattern
Fold the northern edge down to meet the southern edge of the paper, crease and unfold. (There should now be a east/west valley fold running through the center of your paper)
Fold the eastern edge to meet the western edge, crease and unfold. (There should now be two valley folds, running east/west and north/south, that intersect in the center of the paper)
Flip the paper over. (There should now be two mountain folds, running east/west and north/south, that intersect in the center of the paper)
Fold the northwest corner so it meets the southeastern corner, crease and unfold. (along with the intersecting mountain folds, there should now be a diagonal fold running northeast/southwest)
Repeat the last step in the other direction, folding the northeast corner to the southwestern corner, crease and unfold. (there should now be four creases on the paper, two mountain running east/west and north/south, and two valley diagonal folds that all intersect in the center of the paper.)
Collapse the pattern
Turn the paper 45 degrees. (now the previously diagonal folds should be running east/west and north/south, and each of the corners will now be positioned in each of the cardinal directions.)
Lift the portion of the paper south of the east/west crease so that it is perpendicular to the rest of the paper. (The paper should now be divided into two, main, north and south, triangular sections at a 90 degree angle to each other, each section should be divided into two right triangles, which are farther divided into smaller triangles by diagonal mountain folds.
Fold the eastern corner along the mountain fold dividing the southeastern triangular quadrant so that it meets the southern corner. (Now the SouthSouthwest, south southeast, and southeast east sub sections should be folded together, the over all paper may resemble a upside down tetrahedron, with the top open and each side separated into two sub triangles, with the layered flap created in this step on the east section of the southern side)
Fold the southern subsection of the east side of the tetrahedron on to the layered triangle created in the last step. (The east half of the paper should now consist of a four layered right triangle made up of two individual triangular flaps, and the western half may resemble a upside down pyramidal pocket, whose north and south corners are raised.
While holding the eastern layered triangle, find the western crease on the pocket, push it towards the center of the model so that the north and south corners of the pocket meet at the western most corner of the model, and a second layered triangle, composed of two individual flaps, is formed. (the water bomb base is now complete!, it should be a four layer isosceles triangle with a crease separating it into two right triangles, each made up from two triangular flaps.