An apparatus, comprising: an outer tank having an open top, a tank wall, and a closed bottom; an outer ringwall extending to a first height from the bottom, the outer ringwall spaced apart from the tank wall such that a first annular space is defined between the outer ringwall and the tank wall, and a second annular space is defined between the outer ringwall an inner ringwall; the inner ringwall extending to a second height from the bottom, and cylindrical space defined inside the inner ringwall; a cylindrical air conduit extending to a third height through the bottom and into the cylindrical space; a pod disposed at least partially into the cylindrical space, the pod comprising a closed top, a wall, and a bottom defining a closed gas-filled chamber having a cylindrical recess receiving the air conduit therein, the wall extending past the bottom to define an open displacement chamber, the wall separated from the inner ringwall by a first annular gap; and an inner riser at least partially disposed in the second annular space, the inner riser having a closed top having upper and lower surfaces, a wall, and an open bottom, the lower surface of the inner riser resting on the inner ringwall, the wall separated from the inner ringwall by a second annular gap, and separated from the outer ringwall by a third annular gap; an outer riser at least partially disposed in the first annular space and having a closed top, a wall, and an open bottom, the closed top resting on the outer ringwall and onto the inner riser, the wall separated from the outer ringwall by a fourth annular gap, and separated from the tank wall by a fifth annular gap, wherein the outer tank is at least partially filled with a liquid, and wherein the first annular gap, the third annular gap, and the fifth annular gap are substantially full of the liquid, and the displacement chamber, the second annular gap, and the fourth annular gap are substantially free of the liquid, such that the pod, inner riser, and outer riser are positively buoyant and capable of traveling upward relative to the outer tank. an outer tank having an open top, a tank wall, and a closed bottom; an outer ringwall extending to a first height from the bottom, the outer ringwall spaced apart from the tank wall such that a first annular space is defined between the outer ringwall and the tank wall, and a second annular space is defined between the outer ringwall an inner ringwall; the inner ringwall extending to a second height from the bottom, and cylindrical space defined inside the inner ringwall; a cylindrical air conduit extending to a third height through the bottom and into the cylindrical space; a pod disposed at least partially into the cylindrical space, the pod comprising a closed top, a wall, and a bottom defining a closed gas-filled chamber having a cylindrical recess receiving the air conduit therein, the wall extending past the bottom to define an open displacement chamber, the wall separated from the inner ringwall by a first annular gap; and an inner riser at least partially disposed in the second annular space, the inner riser having a closed top having upper and lower surfaces, a wall, and an open bottom, the lower surface of the inner riser resting on the inner ringwall, the wall separated from the inner ringwall by a second annular gap, and separated from the outer ringwall by a third annular gap; an outer riser at least partially disposed in the first annular space and having a closed top, a wall, and an open bottom, the closed top resting on the outer ringwall and onto the inner riser, the wall separated from the outer ringwall by a fourth annular gap, and separated from the tank wall by a fifth annular gap, wherein the outer tank is at least partially filled with a liquid, and wherein the first annular gap, the third annular gap, and the fifth annular gap are substantially full of the liquid, and the displacement chamber, the second annular gap, and the fourth annular gap are substantially free of the liquid, such that the pod, inner riser, and outer riser are positively buoyant and capable of traveling upward relative to the outer tank.