A shale shaker comprising: a basket including a stack of at least three screen decks (8, 8a, 8b), each deck having a screening surface (9, 9a, 9b), with the screening surface of each deck spaced apart and superposed one above the other to constitute upper, middle and lowest screen decks, characterized in that: the screening surfaces (9, 9a, 9b) of each of the screen decks (8, 8a, 8b) have a lower, feed receiving, end that is proximal to a fluid retaining wall of the basket and a distal, higher, solids discharge end; wherein the screening surfaces (9, 9a, 9b) of each of the screen decks (8, 8a, 8b) are each spaced apart, one from the next in the stack of three, by a spacing of from 20mm to 120mm, optionally of from 30mm to 80mm or optionally of from 40mm to 60mm; at their feed receiving ends; the fluid retaining wall and the feed receiving end of the screening surface of the lowest screen deck defines a pond region; the screening surfaces of the upper, middle and lowest screen decks are in sealing engagement with the fluid retaining wall; whereby the apparatus is configured such that when a drill cuttings and drilling mud mixture feed is provided to the feed receiving end of the at least three screen decks a pond of solids and liquid is formed in the pond region that immerses a portion of the screening surface at the feed receiving end of at least the lowest and middle screen decks; and the shale shaker is configured to be operated with a feed rate so as to maintain the pond whilst screening the drill cuttings and drilling mud mixture feed on the screening surfaces; so as to allow full or substantially full successive screening in turn of the drill cuttings and drilling mud mixture through the middle and lowest screen deck of the portion of drill cuttings and drilling mud mixture feed that passes through the screening surface of the upper screen deck. a basket including a stack of at least three screen decks (8, 8a, 8b), each deck having a screening surface (9, 9a, 9b), with the screening surface of each deck spaced apart and superposed one above the other to constitute upper, middle and lowest screen decks, characterized in that: the screening surfaces (9, 9a, 9b) of each of the screen decks (8, 8a, 8b) have a lower, feed receiving, end that is proximal to a fluid retaining wall of the basket and a distal, higher, solids discharge end; wherein the screening surfaces (9, 9a, 9b) of each of the screen decks (8, 8a, 8b) are each spaced apart, one from the next in the stack of three, by a spacing of from 20mm to 120mm, optionally of from 30mm to 80mm or optionally of from 40mm to 60mm; at their feed receiving ends; the fluid retaining wall and the feed receiving end of the screening surface of the lowest screen deck defines a pond region; the screening surfaces of the upper, middle and lowest screen decks are in sealing engagement with the fluid retaining wall; whereby the apparatus is configured such that when a drill cuttings and drilling mud mixture feed is provided to the feed receiving end of the at least three screen decks a pond of solids and liquid is formed in the pond region that immerses a portion of the screening surface at the feed receiving end of at least the lowest and middle screen decks; and the shale shaker is configured to be operated with a feed rate so as to maintain the pond whilst screening the drill cuttings and drilling mud mixture feed on the screening surfaces; so as to allow full or substantially full successive screening in turn of the drill cuttings and drilling mud mixture through the middle and lowest screen deck of the portion of drill cuttings and drilling mud mixture feed that passes through the screening surface of the upper screen deck. the screening surfaces (9, 9a, 9b) of each of the screen decks (8, 8a, 8b) have a lower, feed receiving, end that is proximal to a fluid retaining wall of the basket and a distal, higher, solids discharge end; wherein the screening surfaces (9, 9a, 9b) of each of the screen decks (8, 8a, 8b) are each spaced apart, one from the next in the stack of three, by a spacing of from 20mm to 120mm, optionally of from 30mm to 80mm or optionally of from 40mm to 60mm; at their feed receiving ends; the fluid retaining wall and the feed receiving end of the screening surface of the lowest screen deck defines a pond region; the screening surfaces of the upper, middle and lowest screen decks are in sealing engagement with the fluid retaining wall; whereby the apparatus is configured such that when a drill cuttings and drilling mud mixture feed is provided to the feed receiving end of the at least three screen decks a pond of solids and liquid is formed in the pond region that immerses a portion of the screening surface at the feed receiving end of at least the lowest and middle screen decks; and the shale shaker is configured to be operated with a feed rate so as to maintain the pond whilst screening the drill cuttings and drilling mud mixture feed on the screening surfaces; so as to allow full or substantially full successive screening in turn of the drill cuttings and drilling mud mixture through the middle and lowest screen deck of the portion of drill cuttings and drilling mud mixture feed that passes through the screening surface of the upper screen deck.