diff --git a/src/virtualship/expedition/input_data.py b/src/virtualship/expedition/input_data.py
index 7af9ef72..a280c477 100644
--- a/src/virtualship/expedition/input_data.py
+++ b/src/virtualship/expedition/input_data.py
@@ -97,7 +97,9 @@ def _load_default_fieldset(cls, directory: str | Path) -> FieldSet:
 
         # make depth negative
         for g in fieldset.gridset.grids:
-            g.depth = -g.depth
+            g._depth = (
+                -g._depth
+            )  # TODO maybe add a grid.negate_depth() method in Parcels?
 
         # add bathymetry data
         bathymetry_file = directory.joinpath("bathymetry.nc")
@@ -137,7 +139,7 @@ def _load_drifter_fieldset(cls, directory: str | Path) -> FieldSet:
 
         # make depth negative
         for g in fieldset.gridset.grids:
-            g.depth = -g.depth
+            g._depth = -g._depth
 
         # read in data already
         fieldset.computeTimeChunk(0, 1)
@@ -169,7 +171,7 @@ def _load_argo_float_fieldset(cls, directory: str | Path) -> FieldSet:
         # make depth negative
         for g in fieldset.gridset.grids:
             if max(g.depth) > 0:
-                g.depth = -g.depth
+                g._depth = -g._depth
 
         # read in data already
         fieldset.computeTimeChunk(0, 1)